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

  $ rm -rf a
  $ hg init a
  $ cd a
  $ echo a > a
  $ hg ci -Am0
  adding a
  $ hg tag t1 # 1
  $ hg tag --remove t1 # 2

  $ hg co 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg tag -f -r0 t1
  $ hg tags
  tip                                3:a49829c4fc11
  t1                                 0:f7b1eb17ad24

  $ cd ..