Merge branch 'jk/grep-no-index-fix'

The code to parse the command line "git grep <patterns>... <rev>
[[--] <pathspec>...]" has been cleaned up, and a handful of bugs
have been fixed (e.g. we used to check "--" if it is a rev).

* jk/grep-no-index-fix:
  grep: treat revs the same for --untracked as for --no-index
  grep: do not diagnose misspelt revs with --no-index
  grep: avoid resolving revision names in --no-index case
  grep: fix "--" rev/pathspec disambiguation
  grep: re-order rev-parsing loop
  grep: do not unnecessarily query repo for "--"
  grep: move thread initialization a little lower