feat(pack): allow running update() without Internet connection

Problem: There is now way to run `update()` without Internet connection
  while there are some workflows that do not require it. Like "switch
  plugin version" and "revert latest update".

Solution: Add `opts.offline` to `update()`. This also allows now to
  treat `vim.pack.update(nil, { offline = true })` as a way to
  interactively explore currently installed plugins.

runtime/doc/pack.txt

@@ -288,12 +288,18 @@ Another approach is to utilize Git's `insteadOf` configuration:
   These sources will be used verbatim in |vim.pack-lockfile|, so reusing the
   config on different machine will require the same Git configuration.
 
+Explore installed plugins ~
+• `vim.pack.update(nil, { offline = true })`
+• Navigate between plugins with `[[` and `]]`. List them with `gO`
+  (|vim.lsp.buf.document_symbol()|).
+
 Switch plugin's version and/or source ~
 • Update 'init.lua' for plugin to have desired `version` and/or `src`. Let's
   say, the switch is for plugin named 'plugin1'.
 • |:restart|. The plugin's state on disk (revision and/or tracked source) is
   not yet changed. Only plugin's `version` in |vim.pack-lockfile| is updated.
-• Execute `vim.pack.update({ 'plugin1' })`. The plugin's source is updated.
+• Execute `vim.pack.update({ 'plugin1' })`. The plugin's source is updated. If
+  only switching version, use `{ offline = true }` option table.
 • Review changes and either confirm or discard them. If discarded, revert
   `version` change in 'init.lua' as well or you will be prompted again next
   time you run |vim.pack.update()|.
@@ -315,7 +321,8 @@ Revert plugin after an update ~
     locate the revisions before the latest update, and (carefully) adjust
     current lockfile to have those revisions.
 • |:restart|.
-• `vim.pack.update({ 'plugin' }, { target = 'lockfile' })`. Read and confirm.
+• `vim.pack.update({ 'plugin' }, { offline = true, target = 'lockfile' })`.
+  Read and confirm.
 
 Synchronize config across machines ~
 • On main machine:
@@ -491,6 +498,8 @@ update({names}, {opts})                                    *vim.pack.update()*
       • {opts}   (`table?`) A table with the following fields:
                  • {force}? (`boolean`) Whether to skip confirmation and make
                    updates immediately. Default `false`.
+                 • {offline}? (`boolean`) Whether to skip downloading new
+                   updates. Default: `false`.
                  • {target}? (`string`) How to compute a new plugin revision.
                    One of:
                    • "version" (default) - use latest revision matching

runtime/lua/vim/pack.lua

@@ -88,6 +88,12 @@
 ---  These sources will be used verbatim in |vim.pack-lockfile|, so reusing
 ---  the config on different machine will require the same Git configuration.
 ---
+---Explore installed plugins ~
+---
+---- `vim.pack.update(nil, { offline = true })`
+---- Navigate between plugins with `[[` and `]]`. List them with `gO`
+---  (|vim.lsp.buf.document_symbol()|).
+---
 ---Switch plugin's version and/or source ~
 ---
 ---- Update 'init.lua' for plugin to have desired `version` and/or `src`.
@@ -95,6 +101,7 @@
 ---- |:restart|. The plugin's state on disk (revision and/or tracked source)
 ---  is not yet changed. Only plugin's `version` in |vim.pack-lockfile| is updated.
 ---- Execute `vim.pack.update({ 'plugin1' })`. The plugin's source is updated.
+---  If only switching version, use `{ offline = true }` option table.
 ---- Review changes and either confirm or discard them. If discarded, revert
 ---  `version` change in 'init.lua' as well or you will be prompted again next time
 ---  you run |vim.pack.update()|.
@@ -119,7 +126,8 @@
 ---      locate the revisions before the latest update, and (carefully) adjust
 ---      current lockfile to have those revisions.
 ---- |:restart|.
----- `vim.pack.update({ 'plugin' }, { target = 'lockfile' })`. Read and confirm.
+---- `vim.pack.update({ 'plugin' }, { offline = true, target = 'lockfile' })`.
+---  Read and confirm.
 ---
 ---Synchronize config across machines ~
 ---
@@ -1152,6 +1160,8 @@ end
 --- @inlinedoc
 --- @field force? boolean Whether to skip confirmation and make updates immediately. Default `false`.
 ---
+--- @field offline? boolean Whether to skip downloading new updates. Default: `false`.
+---
 --- How to compute a new plugin revision. One of:
 ---     - "version" (default) - use latest revision matching `version` from plugin specification.
 ---     - "lockfile" - use revision from the lockfile. Useful for reverting or performing controlled
@@ -1191,7 +1201,7 @@ end
 --- @param opts? vim.pack.keyset.update
 function M.update(names, opts)
   vim.validate('names', names, vim.islist, true, 'list')
-  opts = vim.tbl_extend('force', { force = false, target = 'version' }, opts or {})
+  opts = vim.tbl_extend('force', { force = false, offline = false, target = 'version' }, opts or {})
 
   local plug_list = plug_list_from_names(names)
   if #plug_list == 0 then
@@ -1217,7 +1227,7 @@ function M.update(names, opts)
     end
 
     -- Fetch
-    if not opts._offline then
+    if not opts.offline then
       -- Using '--tags --force' means conflicting tags will be synced with remote
       local args = { 'fetch', '--quiet', '--tags', '--force', '--recurse-submodules=yes', 'origin' }
       git_cmd(args, p.path)
@@ -1237,8 +1247,8 @@ function M.update(names, opts)
       trigger_event(p, 'PackChanged', 'update')
     end
   end
-  local progress_title = opts.force and (opts._offline and 'Applying updates' or 'Updating')
-    or 'Downloading updates'
+  local progress_title = opts.force and (opts.offline and 'Applying updates' or 'Updating')
+    or (opts.offline and 'Computing updates' or 'Downloading updates')
   run_list(plug_list, do_update, progress_title)
 
   if needs_lock_write then

runtime/lua/vim/pack/_lsp.lua

@@ -157,7 +157,7 @@ end
 
 local commands = {
   update_plugin = function(plug_data)
-    vim.pack.update({ plug_data.name }, { force = true, _offline = true })
+    vim.pack.update({ plug_data.name }, { force = true, offline = true })
   end,
   skip_update_plugin = function(_) end,
   delete_plugin = function(plug_data)

test/functional/plugin/pack_spec.lua

@@ -1765,6 +1765,28 @@ describe('vim.pack', function()
       assert_origin(basic_src)
     end)
 
+    it('can do offline update', function()
+      local defbranch_path = pack_get_plug_path('defbranch')
+      local defbranch_lua_file = vim.fs.joinpath(defbranch_path, 'lua', 'defbranch.lua')
+
+      n.exec_lua(function()
+        vim.pack.add({ { src = repos_src.defbranch, version = 'main' } })
+      end)
+
+      track_nvim_echo()
+
+      eq('return "defbranch dev"', fn.readblob(defbranch_lua_file))
+      n.exec_lua(function()
+        vim.pack.update({ 'defbranch' }, { offline = true })
+      end)
+
+      -- There should be no progress report about downloading updates
+      assert_progress_report('Computing updates', { 'defbranch' })
+
+      n.exec('write')
+      eq('return "defbranch main"', fn.readblob(defbranch_lua_file))
+    end)
+
     it('shows progress report', function()
       track_nvim_echo()
       exec_lua(function()