completion: use __gitcomp_builtin in _git_grep

The new completable options are:

--after-context=
--before-context=
--color
--context
--exclude-standard
--quiet
--recurse-submodules
--textconv

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1565,21 +1565,7 @@ _git_grep ()
 
 	case "$cur" in
 	--*)
-		__gitcomp "
-			--cached
-			--text --ignore-case --word-regexp --invert-match
-			--full-name --line-number
-			--extended-regexp --basic-regexp --fixed-strings
-			--perl-regexp
-			--threads
-			--files-with-matches --name-only
-			--files-without-match
-			--max-depth
-			--count
-			--and --or --not --all-match
-			--break --heading --show-function --function-context
-			--untracked --no-index
-			"
+		__gitcomp_builtin grep
 		return
 		;;
 	esac