test-grep: add tests for --follow with/without --diff and/or paths
This tests the behavior of cmdutil.walkchangerevs().
I'm going to rewrite cmdutil.walkchangerevs() to leverage the "log -f"
logic, but the code coverage looked quite small. And the history traversal
of "grep -f" goes wrong in various ways.
from __future__ import absolute_import
import hashlib
try:
from ..thirdparty import sha1dc
sha1 = sha1dc.sha1
except (ImportError, AttributeError):
sha1 = hashlib.sha1