transport-helper: fix sync issue on crashes

When a remote helper crashes while pushing we should revert back to the
state before the push, however, it's possible that `git fast-export`
already finished its job, and therefore has exported the marks already.

This creates a synchronization problem because from that moment on
`git fast-{import,export}` will have marks that the remote helper is not
aware of and all further commands fail (if those marks are referenced).

The fix is to tell `git fast-export` to export to a temporary file, and
only after the remote helper has finishes successfully, move to the
final destination.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5801-remote-helpers.sh

@@ -220,6 +220,20 @@ test_expect_success 'push update refs failure' '
 	)
 '
 
+clean_mark () {
+	cut -f 2 -d ' ' "$1" |
+	git cat-file --batch-check |
+	grep commit |
+	sort >$(basename "$1")
+}
+
+cmp_marks () {
+	test_when_finished "rm -rf git.marks testgit.marks" &&
+	clean_mark ".git/testgit/$1/git.marks" &&
+	clean_mark ".git/testgit/$1/testgit.marks" &&
+	test_cmp git.marks testgit.marks
+}
+
 test_expect_success 'proper failure checks for fetching' '
 	(GIT_REMOTE_TESTGIT_FAILURE=1 &&
 	export GIT_REMOTE_TESTGIT_FAILURE &&
@@ -232,7 +246,11 @@ test_expect_success 'proper failure checks for fetching' '
 
 test_expect_success 'proper failure checks for pushing' '
 	(cd local &&
-	test_must_fail env GIT_REMOTE_TESTGIT_FAILURE=1 git push --all
+	git checkout -b crash master &&
+	echo crash >>file &&
+	git commit -a -m crash &&
+	test_must_fail env GIT_REMOTE_TESTGIT_FAILURE=1 git push --all &&
+	cmp_marks origin
 	)
 '