Merge branch 'jc/maint-grep-untracked-exclude' into jc/grep-untracked-exclude

* jc/maint-grep-untracked-exclude:
grep: teach --untracked and --exclude-standard options
grep --no-index: don't use git standard exclusions
grep: do not use --index in the short usage output

Conflicts:
Documentation/git-grep.txt
builtin/grep.c