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Migration mode does not work on Windows if legacy script starts with `#!/bin/sh`

See original GitHub issue

Repro steps:

Create a git repo on windows, install git-lfs. It will add pre-push hook with this content:

#!/bin/sh
command -v git-lfs >/dev/null 2>&1 || { echo >&2 "\nThis repository is configured for Git LFS but 'git-lfs' was not found on your path. If you no longer wish to use Git LFS, remove this hook by deleting .git/hooks/pre-push.\n"; exit 2; }
git lfs pre-push "$@"

Install pre-commit and call pre-commit install -t pre-push It will rename pre-push to pre-push.legacy, command output:

pre-commit installed at .git\hooks\pre-commit
Running in migration mode with existing hooks at .git\hooks\pre-push.legacy
Use -f to use only pre-commit.
pre-commit installed at .git\hooks\pre-push

Try pushing with git push It will fail with:

An unexpected error has occurred: ExecutableNotFoundError: Executable `/bin/sh` not found
Check the log at C:\Users\<username>\.cache\pre-commit\pre-commit.log
error: failed to push some refs to <repo url>

Expected: git push will work and will successfully call legacy git-lfs hook

Actual: git push fails

Log file content:

version information

pre-commit version: 2.7.1
sys.version:
    3.7.7 (default, May  6 2020, 11:45:54) [MSC v.1916 64 bit (AMD64)]
sys.executable: d:\envs\polymer\python.exe
os.name: nt
sys.platform: win32

error information

An unexpected error has occurred: ExecutableNotFoundError: Executable `/bin/sh` not found

Traceback (most recent call last):
  File "d:\envs\polymer\lib\site-packages\pre_commit\error_handler.py", line 63, in error_handler
    yield
  File "d:\envs\polymer\lib\site-packages\pre_commit\main.py", line 369, in main
    args=args.rest[1:],
  File "d:\envs\polymer\lib\site-packages\pre_commit\commands\hook_impl.py", line 211, in hook_impl
    retv, stdin = _run_legacy(hook_type, hook_dir, args)
  File "d:\envs\polymer\lib\site-packages\pre_commit\commands\hook_impl.py", line 42, in _run_legacy
    cmd = normalize_cmd((legacy_hook, *args))
  File "d:\envs\polymer\lib\site-packages\pre_commit\parse_shebang.py", line 82, in normalize_cmd
    exe = normexe(cmd[0])
  File "d:\envs\polymer\lib\site-packages\pre_commit\parse_shebang.py", line 61, in normexe
    _error('not found')
  File "d:\envs\polymer\lib\site-packages\pre_commit\parse_shebang.py", line 51, in _error
    raise ExecutableNotFoundError(f'Executable `{orig}` {msg}')
pre_commit.parse_shebang.ExecutableNotFoundError: Executable `/bin/sh` not found