grep: reuse getrenamedfn() from scmutil
My motivation is to reduce uses of filectx.renamed(). Reusing
scmutil.getrenamedfn() also means that we get some caching of copy
information per file and revision. I don't think that matters for `hg
grep` (I doubt it speeds up significantly, and I doubt it wastes
significant memory), but I'm not sure.
Differential Revision: https://phab.mercurial-scm.org/D6022