def _rev_exists(rev: str) -> bool:
    return not subprocess.call(('git', 'rev-list', '--quiet', rev))

def _rev_exists(rev: str) -> bool:
    return not subprocess.call(
        ('git', 'rev-parse', '--verify', '--quiet', rev),
        stdout=subprocess.DEVNULL,
        stderr=subprocess.DEVNULL,
    )

def _get_diff() -> bytes:
    _, out, _ = cmd_output_b(
        'git', 'diff', '--no-ext-diff', '--no-textconv', '--ignore-submodules',
        check=False,
    )
    return out

def _get_diff(files: tuple[str, ...] = ()) -> bytes:
    cmd = ['git', 'diff', '--no-ext-diff', '--no-textconv', '--ignore-submodules']
    if files:
        cmd.extend(['--', *files])
    _, out, _ = cmd_output_b(*cmd, check=False)
    return out

# filenames from classifier are available before pass_filenames check
filenames = tuple(classifier.filenames_for_hook(hook))
...
if not hook.pass_filenames:
    # Can't scope diff - hook may modify any file
    diff_after = _get_diff()
else:
    # Safe to scope diff to the files hook received
    diff_after = _get_diff(filenames)

# Test _rev_exists performance
python3 -c "
import time, subprocess
for cmd, name in [
    (['git', 'rev-list', '--quiet', 'HEAD'], 'current'),
    (['git', 'rev-parse', '--verify', '--quiet', 'HEAD'], 'fix'),
]:
    start = time.perf_counter()
    subprocess.call(cmd, stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL)
    print(f'{name}: {(time.perf_counter()-start)*1000:.1f}ms')
"

# Test _get_diff performance
python3 -c "
import time, subprocess
for scope in ['', '-- path/to/file.py']:
    cmd = ['git', 'diff', '--no-ext-diff', '--no-textconv', '--ignore-submodules'] + scope.split()
    start = time.perf_counter()
    subprocess.run(cmd, capture_output=True)
    print(f'{scope or 'full tree'}: {(time.perf_counter()-start)*1000:.1f}ms')
"

(sadly I can't share)