feat: make better error message if try-repo fails because there are no commits on the repo.

pre_commit/git.py

@@ -168,7 +168,19 @@ def get_changed_files(old: str, new: str) -> list[str]:
 
 
 def head_rev(remote: str) -> str:
-    _, out, _ = cmd_output('git', 'ls-remote', '--exit-code', remote, 'HEAD')
+    try:
+        _, out, _ = cmd_output(
+            'git', 'ls-remote', '--exit-code',
+            remote, 'HEAD',
+        )
+    except CalledProcessError as e:
+        if e.returncode == 2:
+            raise FatalError(
+                f'repo {remote} found but appears to have no commits. '
+                f'Make a commit in {remote} to use it',
+            )
+        raise e
+
     return out.split()[0]
 
 

testing/fixtures.py

@@ -42,9 +42,10 @@ def git_dir(tempdir_factory):
     return path
 
 
-def make_repo(tempdir_factory, repo_source):
+def make_repo(tempdir_factory, repo_source, commits=True):
     path = git_dir(tempdir_factory)
     copy_tree_to_path(get_resource_path(repo_source), path)
+    if commits:
         cmd_output('git', 'add', '.', cwd=path)
         git_commit(msg=make_repo.__name__, cwd=path)
     return path

tests/commands/try_repo_test.py

@@ -5,10 +5,12 @@ import re
 import time
 from unittest import mock
 
+import pytest
 import re_assert
 
 from pre_commit import git
 from pre_commit.commands.try_repo import try_repo
+from pre_commit.errors import FatalError
 from pre_commit.util import cmd_output
 from testing.auto_namedtuple import auto_namedtuple
 from testing.fixtures import git_dir
@@ -98,6 +100,21 @@ 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_no_commits(cap_out, tempdir_factory):
+    repo = make_repo(
+        tempdir_factory,
+        'modified_file_returns_zero_repo',
+        commits=False,
+    )
+
+    with cwd(git_dir(tempdir_factory)):
+        bare_repo = os.path.join(repo, '.git')
+        # previously crashed attempting modification changes
+        with pytest.raises(FatalError) as e:
+            try_repo(try_repo_opts(bare_repo, hook='bash_hook'))
+            assert 'appears to have no commits' in e.value
+
+
 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)):