Pull upstream force-pushed Git tags

Resolves wbthomason#1100. Pull upstream tags even if they
were force-pushed. Git version 2.20 changed the behavior
of fetching tags so that non-fast-forward updates would fail.
Some plugin authors, like `nvim-tree/nvim-tree.lua` use a
tag named `nightly` that is force-pushed with latest updates
to provide bleeding-edge changes.

By adding an explicit refspec to the `git fetch` command
line with a leading plus-sign ('+') Git will always update
the tag to the new commit, regardless of whether it was
force-pushed upstream.

See https://www.git-scm.com/docs/git-fetch#Documentation/git-fetch.txt-ltrefspecgt
for details (paragraph beginning "Until Git version 2.20...").

lua/packer/plugin_types/git.lua

@@ -380,8 +380,18 @@ git.setup = function(plugin)
         options = { env = git.job_env },
       }
 
+      -- If a tag is specified, name its refspec on the update
+      -- or fetch command line with a leading plus-sign ('+') to ensure that
+      -- upstream commits are pull/fetch-ed even if upstream force-pushed.
+      -- Not needed for branches, because in that case pulling force-pushed
+      -- commits is the default.
+      local explicit_refspec = ''
+      if plugin.tag then
+        explicit_refspec = fmt(' origin +refs/tags/%s', plugin.tag)
+      end
+
       if needs_checkout then
-        r:and_then(await, jobs.run(config.exec_cmd .. config.subcommands.fetch, update_opts))
+        r:and_then(await, jobs.run(fetch_cmd .. explicit_refspec, update_opts))
         r:and_then(await, handle_checkouts(plugin, install_to, disp, opts))
         local function merge_output(res)
           if res.output ~= nil then
@@ -406,13 +416,13 @@ git.setup = function(plugin)
 
       if opts.preview_updates then
         disp:task_update(plugin_name, 'fetching updates...')
-        r:and_then(await, jobs.run(fetch_cmd, update_opts))
+        r:and_then(await, jobs.run(fetch_cmd .. explicit_refspec, update_opts))
       elseif opts.pull_head then
         disp:task_update(plugin_name, 'pulling updates from head...')
         r:and_then(await, jobs.run(update_head_cmd, update_opts))
       else
         disp:task_update(plugin_name, 'pulling updates...')
-        r:and_then(await, jobs.run(update_cmd, update_opts)):and_then(await, jobs.run(submodule_cmd, update_opts))
+        r:and_then(await, jobs.run(update_cmd .. explicit_refspec, update_opts)):and_then(await, jobs.run(submodule_cmd, update_opts))
       end
       r:map_err(function(err)
         plugin.output = { err = vim.list_extend(update_info.err, update_info.output), data = {} }