bash: Support --add completion to git-config.

We've recently added --add as an argument to git-config, but I
missed putting it into the earlier round of git-config updates
within the bash completion.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

contrib/completion/git-completion.bash

@@ -719,7 +719,7 @@ _git_config ()
 		__gitcomp "
 			--global --list --replace-all
 			--get --get-all --get-regexp
-			--unset --unset-all
+			--add --unset --unset-all
 			"
 		return
 		;;