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/explorer/watch.lua

@@ -4,11 +4,7 @@ local Watcher = require("nvim-tree.watcher").Watcher
 
 local M = {}
 
-local function is_git(path)
-  return vim.fn.fnamemodify(path, ":t") == ".git"
-end
-
-local IGNORED_PATHS = {
+M.ignore_dirs = {
   -- disable watchers on kernel filesystems
   -- which have a lot of unwanted events
   "/sys",
@@ -16,13 +12,11 @@ local IGNORED_PATHS = {
   "/dev",
 }
 
-local function is_folder_ignored(path)
-  for _, folder in ipairs(IGNORED_PATHS) do
-    if vim.startswith(path, folder) then
-      return true
-    end
-  end
+function M.ignore_dir(path)
+  table.insert(M.ignore_dirs, path)
+end
 
+local function is_folder_ignored(path)
   for _, ignore_dir in ipairs(M.ignore_dirs) do
     if vim.fn.match(path, ignore_dir) ~= -1 then
       return true
@@ -44,7 +38,7 @@ function M.create_watcher(node)
     path = node.absolute_path
   end
 
-  if is_git(path) or is_folder_ignored(path) then
+  if is_folder_ignored(path) then
     return nil
   end
 
@@ -74,7 +68,7 @@ end
 function M.setup(opts)
   M.enabled = opts.filesystem_watchers.enable
   M.debounce_delay = opts.filesystem_watchers.debounce_delay
-  M.ignore_dirs = opts.filesystem_watchers.ignore_dirs
+  M.ignore_dirs = vim.tbl_extend("force", M.ignore_dirs, opts.filesystem_watchers.ignore_dirs)
   M.uid = 0
 end