Show better error message when running inside .git

pre_commit/main.py

@@ -122,12 +122,20 @@ def _adjust_args_and_chdir(args):
         args.repo = os.path.abspath(args.repo)
 
     try:
-        os.chdir(git.get_root())
+        toplevel = git.get_root()
     except CalledProcessError:
         raise FatalError(
             'git failed. Is it installed, and are you in a Git repository '
             'directory?',
         )
+    else:
+        if toplevel == '':
+            raise FatalError(
+                'git toplevel unexpectedly empty! make sure you are not '
+                'inside the `.git` directory of your repository.',
+            )
+        else:
+            os.chdir(toplevel)
 
     args.config = os.path.relpath(args.config)
     if args.command in {'run', 'try-repo'}:

tests/main_test.py

@@ -39,6 +39,11 @@ def test_adjust_args_and_chdir_not_in_git_dir(in_tmpdir):
         main._adjust_args_and_chdir(Args())
 
 
+def test_adjust_args_and_chdir_in_dot_git_dir(in_git_dir):
+    with in_git_dir.join('.git').as_cwd(), pytest.raises(FatalError):
+        main._adjust_args_and_chdir(Args())
+
+
 def test_adjust_args_and_chdir_noop(in_git_dir):
     args = Args(command='run', files=['f1', 'f2'])
     main._adjust_args_and_chdir(args)