task: metadata: install: pip install -e .[dev]
patch: diff --git a/pre_commit/git.py b/pre_commit/git.py
index c8faf60..136cefe 100644
--- a/pre_commit/git.py
+++ b/pre_commit/git.py
@@ -141,7 +141,7 @@ def has_diff(*args, **kwargs):
     repo = kwargs.pop('repo', '.')
     assert not kwargs, kwargs
     cmd = ('git', 'diff', '--quiet', '--no-ext-diff') + args
-    return cmd_output_b(*cmd, cwd=repo, retcode=None)[0]
+    return cmd_output_b(*cmd, cwd=repo, retcode=None)[0] == 1
 
 
 def has_core_hookpaths_set():

problem_statement: [FR][bug?] pre-commit hook repo self-test
Given a repo with `.pre-commit-hooks.yaml` defined (like https://github.com/ansible/ansible-lint), I want to integrate testing of the hooks declared in it.

I can do `pre-commit try-repo https://github.com/ansible/ansible-lint.git` but this hits the remote which I want to avoid. I know that Git itself can work with local fs paths (like `/path/to/.git`) perfectly fine.

So I tried:
<details>
<summary>
  <code>$ <kbd>pre-commit try-repo .git -vvv</kbd></code>
</summary>

```console
➜ pre-commit try-repo .git -vvv
[WARNING] Creating temporary repo with uncommitted changes...
An unexpected error has occurred: CalledProcessError: Command: ('/usr/bin/git', 'add', '-u')
Return code: 128
Expected return code: 0
Output: (none)
Errors: 
    fatal: this operation must be run in a work tree
    

Check the log at ~/.cache/pre-commit/pre-commit.log
```
</details>

The log doesn't reveal anything more than the fact that the Git command failed.
<details>
<summary>
  <code>$ <kbd>cat ~/.cache/pre-commit/pre-commit.log</kbd></code>
</summary>

```console
An unexpected error has occurred: CalledProcessError: Command: ('/usr/bin/git', 'add', '-u')
Return code: 128
Expected return code: 0
Output: (none)
Errors: 
    fatal: this operation must be run in a work tree
    

Traceback (most recent call last):
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/pre_commit/error_handler.py", line 46, in error_handler
    yield
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/pre_commit/main.py", line 296, in main
    return try_repo(args)
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/pre_commit/commands/try_repo.py", line 55, in try_repo
    repo, ref = _repo_ref(tempdir, args.repo, args.ref)
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/pre_commit/commands/try_repo.py", line 45, in _repo_ref
    cmd_output('git', 'add', '-u', cwd=repo, env=env)
  File "~/.pyenv/versions/3.7.1/lib/python3.7/site-packages/pre_commit/util.py", line 153, in cmd_output
    returncode, cmd, retcode, output=(stdout, stderr),
pre_commit.util.CalledProcessError: Command: ('/usr/bin/git', 'add', '-u')
Return code: 128
Expected return code: 0
Output: (none)
Errors: 
    fatal: this operation must be run in a work tree
```
</details>

It must be pretty easy to fix.
repo: pre-commit/pre-commit
test_cmd: pytest --no-header -rA --tb=line --color=no -p no:cacheprovider -W ignore::DeprecationWarning
test_patch: diff --git a/tests/commands/try_repo_test.py b/tests/commands/try_repo_test.py
index 536eb9b..1849c70 100644
--- a/tests/commands/try_repo_test.py
+++ b/tests/commands/try_repo_test.py
@@ -98,6 +98,15 @@ def test_try_repo_relative_path(cap_out, tempdir_factory):
         assert not try_repo(try_repo_opts(relative_repo, hook='bash_hook'))
 
 
+def test_try_repo_bare_repo(cap_out, tempdir_factory):
+    repo = make_repo(tempdir_factory, 'modified_file_returns_zero_repo')
+    with cwd(git_dir(tempdir_factory)):
+        _add_test_file()
+        bare_repo = os.path.join(repo, '.git')
+        # previously crashed attempting modification changes
+        assert not try_repo(try_repo_opts(bare_repo, hook='bash_hook'))
+
+
 def test_try_repo_specific_revision(cap_out, tempdir_factory):
     repo = make_repo(tempdir_factory, 'script_hooks_repo')
     ref = git.head_rev(repo)