i18n: rebase: mark placeholder for translation

Mark placeholder "<branch>" in git-rebase.sh for translation. The string
containing the named placeholder is passed to shell function
error_on_missing_default_upstream in git-parse-remote.sh which uses it
to display a command hint for the user.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -449,7 +449,7 @@ then
 		then
 			. git-parse-remote
 			error_on_missing_default_upstream "rebase" "rebase" \
-				"against" "git rebase <branch>"
+				"against" "git rebase $(gettext '<branch>')"
 		fi
 
 		test "$fork_point" = auto && fork_point=t