fix(pack): skip git stash during install

Problem: Installing plugin is done via `git clone --no-checkout ...`
  (to not end up with default branch code in case of invalid `version`).
  This leaves cloned repo in a state that `git stash` will actually add
  an entry to the stash list. Although not critical, better to not have
  that if possible.

Solution: explicitly skip `git stash` step in checkout during install.

runtime/lua/vim/pack.lua

@@ -654,15 +654,18 @@ end
 --- @async
 --- @param p vim.pack.Plug
 --- @param timestamp string
-local function checkout(p, timestamp)
+--- @param skip_stash? boolean
+local function checkout(p, timestamp, skip_stash)
   infer_revisions(p)
 
-  local stash_cmd = { 'stash', '--quiet' }
-  if git_version > vim.version.parse('2.13') then
-    stash_cmd[#stash_cmd + 1] = '--message'
-    stash_cmd[#stash_cmd + 1] = ('vim.pack: %s Stash before checkout'):format(timestamp)
+  if not skip_stash then
+    local stash_cmd = { 'stash', '--quiet' }
+    if git_version > vim.version.parse('2.13') then
+      stash_cmd[#stash_cmd + 1] = '--message'
+      stash_cmd[#stash_cmd + 1] = ('vim.pack: %s Stash before checkout'):format(timestamp)
+    end
+    git_cmd(stash_cmd, p.path)
   end
-  git_cmd(stash_cmd, p.path)
 
   git_cmd({ 'checkout', '--quiet', p.info.sha_target }, p.path)
 
@@ -691,7 +694,7 @@ 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
 
-    checkout(p, timestamp)
+    checkout(p, timestamp, true)
     p.info.installed = true
 
     trigger_event(p, 'PackChanged', 'install')

test/functional/plugin/pack_spec.lua

@@ -369,6 +369,11 @@ describe('vim.pack', function()
         vim.pack.add({ repos_src.basic })
       end)
       eq(exec_lua('return #_G.event_log'), 0)
+
+      -- Should not create redundant stash entry
+      local basic_path = pack_get_plug_path('basic')
+      local stash_list = system_sync({ 'git', 'stash', 'list' }, { cwd = basic_path }).stdout or ''
+      eq('', stash_list)
     end)
 
     it('passes `data` field through to `opts.load`', function()