grep: make -frREV follow history from the specified revision (BC)
This is close to what "log -frREV" will do, and is backported from
8b4b9ee6001a, "log: make -fr show complete history from the given revs"
except for the "del opts['follow']" bit.
I'm planning to rewrite cmdutil.walkchangerevs() to share the core logic
with logcmdutil, and this is the first step towards that. There are still
many broken tests, but the fundamental behavior should be fixed by this
`hg grep -fr REV` now follows history from the specified `REV`, works in the same way as `hg log -fr REV`. The previous behavior was to limit the search space to `REV` while following the history.