fix(pack)!: adjust install confirm (no error on "No", show names)

Problem: Installation confirmation has several usability issues:
    - Choosing "No" results in a `vim.pack.add()` error. This was by
      design to ensure that all later code that *might* reference
      presumably installed plugin will not get executed. However, this
      is often too restrictive since there might be no such code (like
      if plugin's effects are automated in its 'plugin/' directory).
      Instead the potential code using not installed plugin will throw
      an error.

      No error on "No" will also be useful for planned lockfile repair.

    - List of soon-to-be-installed plugins doesn't mention plugin names.
      This might be confusing if plugins are installed under different
      name.

Solution: Silently drop installation step if user chose "No" and show
  plugin names in confirmation text (together with their pretty aligned
  sources).

runtime/lua/vim/pack.lua

@@ -545,16 +545,24 @@ local function confirm_install(plug_list)
     return true
   end
 
-  local src = {} --- @type string[]
-  for _, p in ipairs(plug_list) do
-    src[#src + 1] = p.spec.src
+  -- Gather pretty aligned list of plugins to install
+  local name_width, name_max_width = {}, 0 --- @type integer[], integer
+  for i, p in ipairs(plug_list) do
+    name_width[i] = api.nvim_strwidth(p.spec.name)
+    name_max_width = math.max(name_max_width, name_width[i])
   end
-  local src_text = table.concat(src, '\n')
-  local confirm_msg = ('These plugins will be installed:\n\n%s\n'):format(src_text)
-  local res = vim.fn.confirm(confirm_msg, 'Proceed? &Yes\n&No\n&Always', 1, 'Question')
-  confirm_all = res == 3
+  local lines = {} --- @type string[]
+  for i, p in ipairs(plug_list) do
+    local pad = (' '):rep(name_max_width - name_width[i] + 1)
+    lines[i] = ('%s%sfrom %s'):format(p.spec.name, pad, p.spec.src)
+  end
+
+  local text = table.concat(lines, '\n')
+  local confirm_msg = ('These plugins will be installed:\n\n%s\n'):format(text)
+  local choice = vim.fn.confirm(confirm_msg, 'Proceed? &Yes\n&No\n&Always', 1, 'Question')
+  confirm_all = choice == 3
   vim.cmd.redraw()
-  return res ~= 2
+  return choice ~= 2
 end
 
 --- @param tags string[]
@@ -662,14 +670,6 @@ end
 
 --- @param plug_list vim.pack.Plug[]
 local function install_list(plug_list, confirm)
-  -- Get user confirmation to install plugins
-  if confirm and not confirm_install(plug_list) then
-    for _, p in ipairs(plug_list) do
-      p.info.err = 'Installation was not confirmed'
-    end
-    return
-  end
-
   local timestamp = get_timestamp()
   --- @async
   --- @param p vim.pack.Plug
@@ -688,7 +688,18 @@ local function install_list(plug_list, confirm)
 
     trigger_event(p, 'PackChanged', 'install')
   end
+
+  -- Install possibly after user confirmation
+  if not confirm or confirm_install(plug_list) then
     run_list(plug_list, do_install, 'Installing plugins')
+  end
+
+  -- Ensure that not installed plugins are absent in lockfile
+  for _, p in ipairs(plug_list) do
+    if not p.info.installed then
+      plugin_lock.plugins[p.spec.name] = nil
+    end
+  end
 end
 
 --- @async
@@ -845,11 +856,6 @@ function M.add(specs, opts)
   if #plugs_to_install > 0 then
     git_ensure_exec()
     install_list(plugs_to_install, opts.confirm)
-    for _, p in ipairs(plugs_to_install) do
-      if not p.info.installed then
-        plugin_lock.plugins[p.spec.name] = nil
-      end
-    end
   end
 
   if needs_lock_write then

test/functional/plugin/pack_spec.lua

@@ -387,18 +387,22 @@ describe('vim.pack', function()
     end)
 
     it('asks for installation confirmation', function()
-      -- Do not confirm installation to see what happens
+      -- Do not confirm installation to see what happens (should not error)
       mock_confirm(2)
 
-      local err = pcall_err(exec_lua, function()
-        vim.pack.add({ repos_src.basic })
+      exec_lua(function()
+        vim.pack.add({ repos_src.basic, { src = repos_src.defbranch, name = 'other-name' } })
       end)
-
-      matches('`basic`:\nInstallation was not confirmed', err)
       eq(false, exec_lua('return pcall(require, "basic")'))
+      eq(false, exec_lua('return pcall(require, "defbranch")'))
 
-      local confirm_msg = 'These plugins will be installed:\n\n' .. repos_src.basic .. '\n'
-      local ref_log = { { confirm_msg, 'Proceed? &Yes\n&No\n&Always', 1, 'Question' } }
+      local confirm_msg_lines = ([[
+        These plugins will be installed:
+
+        basic      from %s
+        other-name from %s]]):format(repos_src.basic, repos_src.defbranch)
+      local confirm_msg = vim.trim(vim.text.indent(0, confirm_msg_lines))
+      local ref_log = { { confirm_msg .. '\n', 'Proceed? &Yes\n&No\n&Always', 1, 'Question' } }
       eq(ref_log, exec_lua('return _G.confirm_log'))
     end)