Merge branch 'mt/grep-cached-untracked'

Merge branch 'mt/grep-cached-untracked'

"git grep --untracked" is meant to be "let's ALSO find in these
files on the filesystem" when looking for matches in the working
tree files, and does not make any sense if the primary search is
done against the index, or the tree objects.  The "--cached" and
"--untracked" options have been marked as mutually incompatible.

* mt/grep-cached-untracked:
  grep: error out if --untracked is used with --cached