fix: 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.

Author: zhimsel

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