i18n: rebase: fix marked string to use eval_gettext variant

The string message marked for translation should use eval_gettext
variant instead of the gettext one, since we want to dollar-substitute
$head_name in the result.

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

git-rebase.sh

@@ -154,7 +154,7 @@ move_to_original_branch () {
 		git symbolic-ref \
 			-m "rebase finished: returning to $head_name" \
 			HEAD $head_name ||
-		die "$(gettext "Could not move back to $head_name")"
+		die "$(eval_gettext "Could not move back to \$head_name")"
 		;;
 	esac
 }