fix(pack)!: do not trigger PackChanged[Pre] kind=update during install

Problem: `PackChanged[Pre]` events with `kind=update` are triggered both
  during plugin's initial installation and after already installed
  plugin was updated.

  It was a deliberate decision to allow writing only a single update
  hook to act as a dedicated "build" entry point (like execute `make` or
  `cargo build —release`). This mimics how other plugin managers have a
  single "build" command.

  This was a result of 'mini.deps' experience with the different
  approach: "update" hooks are not run during install. This proved to be
  confusing as it requires to write two hooks. But also the reason might
  be that 'mini.deps' names it "checkout" hook instead of "update".

  However, the `vim.pack` event approach makes it lower cost to handle
  separate "update" and "install" events. Something like
  `if ev.data.kind == 'install' or ev.data.kind == 'update' then`
  instead of two autocommands.
  Plus this makes clearer separation of events.

Solution: do not trigger `PackChanged[Pre] kind=update` event during
  install.

runtime/lua/vim/pack.lua

@@ -588,14 +588,8 @@ end
 --- @async
 --- @param p vim.pack.Plug
 --- @param timestamp string
---- @param skip_same_sha boolean
-local function checkout(p, timestamp, skip_same_sha)
+local function checkout(p, timestamp)
   infer_states(p)
-  if skip_same_sha and p.info.sha_head == p.info.sha_target then
-    return
-  end
-
-  trigger_event(p, 'PackChangedPre', 'update')
 
   local msg = ('vim.pack: %s Stash before checkout'):format(timestamp)
   git_cmd({ 'stash', '--quiet', '--message', msg }, p.path)
@@ -604,8 +598,6 @@ local function checkout(p, timestamp, skip_same_sha)
 
   plugin_lock.plugins[p.spec.name].rev = p.info.sha_target
 
-  trigger_event(p, 'PackChanged', 'update')
-
   -- (Re)Generate help tags according to the current help files.
   -- Also use `pcall()` because `:helptags` errors if there is no 'doc/'
   -- directory or if it is empty.
@@ -638,12 +630,8 @@ local function install_list(plug_list, confirm)
     -- Prefer revision from the lockfile instead of using `version`
     p.info.sha_target = (plugin_lock.plugins[p.spec.name] or {}).rev
 
-    -- Do not skip checkout even if HEAD and target have same commit hash to
-    -- have new repo in expected detached HEAD state and generated help files.
-    checkout(p, timestamp, false)
+    checkout(p, timestamp)
 
-    -- "Install" event is triggered after "update" event intentionally to have
-    -- it indicate "plugin is installed in its correct initial version"
     trigger_event(p, 'PackChanged', 'install')
   end
   run_list(plug_list, do_install, 'Installing plugins')
@@ -1037,9 +1025,11 @@ function M.update(names, opts)
     -- Compute change info: changelog if any, new tags if nothing to update
     infer_update_details(p)
 
-    -- Checkout immediately if not need to confirm
-    if opts.force then
-      checkout(p, timestamp, true)
+    -- Checkout immediately if no need to confirm
+    if opts.force and p.info.sha_head ~= p.info.sha_target then
+      trigger_event(p, 'PackChangedPre', 'update')
+      checkout(p, timestamp)
+      trigger_event(p, 'PackChanged', 'update')
     end
   end
   local progress_title = opts.force and (opts._offline and 'Applying updates' or 'Updating')
@@ -1070,7 +1060,9 @@ function M.update(names, opts)
     --- @async
     --- @param p vim.pack.Plug
     local function do_checkout(p)
-      checkout(p, timestamp2, true)
+      trigger_event(p, 'PackChangedPre', 'update')
+      checkout(p, timestamp2)
+      trigger_event(p, 'PackChanged', 'update')
     end
     run_list(plugs_to_checkout, do_checkout, 'Applying updates')
 

test/functional/plugin/pack_spec.lua

@@ -674,23 +674,13 @@ describe('vim.pack', function()
       local find_event = make_find_packchanged(log)
       local installpre_basic = find_event('Pre', 'install', 'basic', 'feat-branch', false)
       local installpre_defbranch = find_event('Pre', 'install', 'defbranch', nil, false)
-      local updatepre_basic = find_event('Pre', 'update', 'basic', 'feat-branch', false)
-      local updatepre_defbranch = find_event('Pre', 'update', 'defbranch', nil, false)
-      local update_basic = find_event('', 'update', 'basic', 'feat-branch', false)
-      local update_defbranch = find_event('', 'update', 'defbranch', nil, false)
       local install_basic = find_event('', 'install', 'basic', 'feat-branch', false)
       local install_defbranch = find_event('', 'install', 'defbranch', nil, false)
-      eq(8, #log)
+      eq(4, #log)
 
       -- NOTE: There is no guaranteed installation order among separate plugins (as it is async)
-      eq(true, installpre_basic < updatepre_basic)
-      eq(true, updatepre_basic < update_basic)
-      -- NOTE: "Install" is after "update" to indicate installation at correct version
-      eq(true, update_basic < install_basic)
-
-      eq(true, installpre_defbranch < updatepre_defbranch)
-      eq(true, updatepre_defbranch < update_defbranch)
-      eq(true, update_defbranch < install_defbranch)
+      eq(true, installpre_basic < install_basic)
+      eq(true, installpre_defbranch < install_defbranch)
     end)
 
     it('recognizes several `version` types', function()