feat: support custom $GIT_DIR (#2263)

* feat: Watch $GIT_DIR for git changes, if set

While rarely used, it's possible to set the $GIT_DIR environment
variable to instruct git to use a directory other than `.git`.

This checks if that environment variable is set; if it is, the plugin
will watch that directory. If it's not set, it'll fall back to the
default `.git` directory.

* fix: Don't create two watchers for $GIT_DIR

This will ignore a path for watching if EITHER it's '.git', or the value
of $GIT_DIR (if it's set).

If $GIT_DIR is not set, the vim.env object returns `nil`, which will
never match `path`.

* fix: Attempt to make a relative $GIT_DIR absolute

lua/nvim-tree/git/init.lua

@@ -186,7 +186,8 @@ function M.load_project_status(cwd)
       end)
     end
 
-    watcher = Watcher:new(utils.path_join { project_root, ".git" }, WATCHED_FILES, callback, {
+    local git_dir = vim.env.GIT_DIR or utils.path_join { project_root, ".git" }
+    watcher = Watcher:new(git_dir, WATCHED_FILES, callback, {
       project_root = project_root,
     })
   end