transport-helper: pass --signed-tags=warn-strip to fast-export

Currently, attempting to push a signed tag to a remote helper which uses
fast-export results in the remote helper failing because the default
fast-export action for signed tags is "abort".  This is not helpful for
users because there is no way to pass additional arguments to
fast-export here, either from the remote helper or from the command
line.

In general, the signature will be invalidated by whatever transformation
a remote helper performs on a tag to push it to a repository in a
different format so the correct behaviour is to strip the tag.  Doing
this silently may surprise people, so use "warn-strip" to issue a
warning when a signed tag is encountered.

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

Author: johnkeeping

t/t5801-remote-helpers.sh

@@ -6,6 +6,7 @@
 test_description='Test remote-helper import and export commands'
 
 . ./test-lib.sh
+. "$TEST_DIRECTORY"/lib-gpg.sh
 
 if ! type "${BASH-bash}" >/dev/null 2>&1; then
 	skip_all='skipping remote-testgit tests, bash not available'
@@ -166,4 +167,13 @@ test_expect_success 'push ref with existing object' '
 	compare_refs local dup server dup
 '
 
+test_expect_success GPG 'push signed tag' '
+	(cd local &&
+	git checkout master &&
+	git tag -s -m signed-tag signed-tag &&
+	git push origin signed-tag
+	) &&
+	compare_refs local signed-tag^{} server signed-tag^{}
+'
+
 test_done

transport-helper.c

@@ -410,9 +410,10 @@ static int get_exporter(struct transport *transport,
 	/* we need to duplicate helper->in because we want to use it after
 	 * fastexport is done with it. */
 	fastexport->out = dup(helper->in);
-	fastexport->argv = xcalloc(5 + revlist_args->nr, sizeof(*fastexport->argv));
+	fastexport->argv = xcalloc(6 + revlist_args->nr, sizeof(*fastexport->argv));
 	fastexport->argv[argc++] = "fast-export";
 	fastexport->argv[argc++] = "--use-done-feature";
+	fastexport->argv[argc++] = "--signed-tags=warn-strip";
 	if (data->export_marks)
 		fastexport->argv[argc++] = data->export_marks;
 	if (data->import_marks)