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.
#require docutils gettext
$ $TESTDIR/check-gendoc de
checking for parse errors
Die Dateien werden dem Projektarchiv beim n\xc3\xa4chsten \xc3\x9cbernehmen (commit) hinzugef\xc3\xbcgt. Um dies vorher r\xc3\xbcckg\xc3\xa4ngig zu machen, siehe:hg:`forget`. (esc)
warning: please have a space before :hg: