tests/readlink.py

scmutil: improve documentation of revset APIs

I can never remember the differences between the various revset
APIs. I can never remember that scmutil.revrange() is the one I
want to use from user-facing commands.

Add some documentation to clarify this.

While we're here, the argument name for revrange() is changed to
"specs" because that's what it actually is.

#!/usr/bin/env python

from __future__ import print_function

import errno, os, 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)