tests/readlink.py

histedit: don't crash if commit message is empty

If the commit message is empty, histedit will crash before this patch
because it assumes that `summary.splitlines()` is non-empty. One of
our users at work ran into this crash for a commit that was created by
an internal system.

I don't think we have a good way of testing this because it's hard to
create a commit with an empty commit message. I've added a comment to
help prevent regressions.

Differential Revision: https://phab.mercurial-scm.org/D9325

#!/usr/bin/env python3

from __future__ import absolute_import, print_function

import errno
import os
import sys

for f in sys.argv[1:]:
    try:
        print(f, '->', os.readlink(f))
    except OSError as err:
        if err.errno != errno.EINVAL:
            raise
        print(f, '->', f, 'not a symlink')

sys.exit(0)