transport-helper: add 'signed-tags' capability

This allows a remote helper using the 'export' protocol to specify that
it supports signed tags, changing the handing from 'warn-strip' to
'verbatim'.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-remote-testgit

@@ -38,6 +38,7 @@ do
 			echo "*import-marks $gitmarks"
 			echo "*export-marks $gitmarks"
 		fi
+		test -n "$GIT_REMOTE_TESTGIT_SIGNED_TAGS" && echo "signed-tags"
 		echo
 		;;
 	list)