add git version to error output

pre_commit/error_handler.py

@@ -9,6 +9,7 @@
 from pre_commit import output
 from pre_commit.errors import FatalError
 from pre_commit.store import Store
+from pre_commit.util import cmd_output_b
 from pre_commit.util import force_bytes
 
 
@@ -21,6 +22,9 @@ def _log_and_exit(
     error_msg = f'{msg}: {type(exc).__name__}: '.encode() + force_bytes(exc)
     output.write_line_b(error_msg)
 
+    _, git_version_b, _ = cmd_output_b('git', '--version', retcode=None)
+    git_version = git_version_b.decode(errors='backslashreplace').rstrip()
+
     storedir = Store().directory
     log_path = os.path.join(storedir, 'pre-commit.log')
     with contextlib.ExitStack() as ctx:
@@ -38,6 +42,7 @@ def _log_and_exit(
         _log_line()
         _log_line('```')
         _log_line(f'pre-commit version: {C.VERSION}')
+        _log_line(f'git --version: {git_version}')
         _log_line('sys.version:')
         for line in sys.version.splitlines():
             _log_line(f'    {line}')

tests/error_handler_test.py

@@ -122,6 +122,7 @@ def test_log_and_exit(cap_out, mock_store_dir):
             r'\n'
             r'```\n'
             r'pre-commit version: \d+\.\d+\.\d+\n'
+            r'git --version: git version .+\n'
             r'sys.version:\n'
             r'(    .*\n)*'
             r'sys.executable: .*\n'