completion: complete "git -<TAB>" with short options

"git" itself has completion for its long options and subcommands,
but not for its short options.  Add support for them.

Signed-off-by: Wiktor Mis <mwiktor023@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -3898,7 +3898,7 @@ __git_main ()
 			;;
 		esac
 		case "$cur" in
-		--*)
+		-*)
 			__gitcomp "
 			--paginate
 			--no-pager
@@ -3914,6 +3914,12 @@ __git_main ()
 			--namespace=
 			--no-replace-objects
 			--help
+			-C
+			-P
+			-c
+			-h
+			-p
+			-v
 			"
 			;;
 		*)