dist: Handle git worktrees, which have a .git file instead of dir

This is the second most straight forward stupid way of handling
this (with usiing os.path.exists) as the most stupid obvious way. The
only major advantage is that having .git as something other than a
file or directory still doesn't register.

Fixes: #3378

mesonbuild/scripts/dist.py

@@ -188,7 +188,8 @@ def run(args):
 
     dist_name = build.project_name + '-' + build.project_version
 
-    if os.path.isdir(os.path.join(src_root, '.git')):
+    _git = os.path.join(src_root, '.git')
+    if os.path.isdir(_git) or os.path.isfile(_git):
         names = create_dist_git(dist_name, src_root, bld_root, dist_sub, build.dist_scripts)
     elif os.path.isdir(os.path.join(src_root, '.hg')):
         names = create_dist_hg(dist_name, src_root, bld_root, dist_sub, build.dist_scripts)