fix(#1976): support non-standard $GIT_DIR (#2012)

* Improve $GIT_DIR handling

- Retrieve $GIT_DIR using `git rev-parse --absolute-git-dir`
- Move $GIT_DIR ignore in the project exploration part

Resolves #1976

* Code review

- move norm_path to utils.lua
- fix comment #2012

* add comments for utils.norm_path

* get_git_directory use norm_path from utils

* watcher improvements

---------

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/git/init.lua

@@ -2,6 +2,7 @@ local log = require "nvim-tree.log"
 local utils = require "nvim-tree.utils"
 local git_utils = require "nvim-tree.git.utils"
 local Runner = require "nvim-tree.git.runner"
+local Watch = require "nvim-tree.explorer.watch"
 local Watcher = require("nvim-tree.watcher").Watcher
 local Iterator = require "nvim-tree.iterators.node-iterator"
 local explorer_node = require "nvim-tree.explorer.node"
@@ -164,22 +165,34 @@ function M.load_project_status(cwd)
   }
 
   local watcher = nil
+
   if M.config.filesystem_watchers.enable then
     log.line("watcher", "git start")
 
-    local callback = function(w)
-      log.line("watcher", "git event scheduled '%s'", w.project_root)
-      utils.debounce("git:watcher:" .. w.project_root, M.config.filesystem_watchers.debounce_delay, function()
-        if w.destroyed then
-          return
-        end
-        reload_tree_at(w.project_root)
-      end)
-    end
+    local git_directory = git_utils.get_git_directory(project_root)
 
-    watcher = Watcher:new(utils.path_join { project_root, ".git" }, WATCHED_FILES, callback, {
-      project_root = project_root,
-    })
+    if git_directory == nil then
+      log.line("watcher", "could not found the location of .git folder")
+    else
+      local callback = function(w)
+        log.line("watcher", "git event scheduled '%s'", w.project_root)
+        utils.debounce("git:watcher:" .. w.project_root, M.config.filesystem_watchers.debounce_delay, function()
+          if w.destroyed then
+            return
+          end
+          reload_tree_at(w.project_root)
+        end)
+      end
+
+      -- Add GIT_DIR to the list of directory to ignore
+      -- local base_gitdir = utils.path_basename(git_directory)
+      -- Watch.ignore_dir(base_gitdir)
+      Watch.ignore_dir(git_directory)
+
+      watcher = Watcher:new(git_directory, WATCHED_FILES, callback, {
+        project_root = project_root,
+      })
+    end
   end
 
   M.projects[project_root] = {