t3200: test --set-upstream-to with bogus refs

These tests pass with the current code, but let's make sure
we don't accidentally break the behavior in the future.

Note that our tests expect failure when we try to set the
upstream to or from a missing branch. Technically we are
just munging config here, so we do not need the refs to
exist. But seeing that they do exist is a good check that
the user has not made a typo.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3200-branch.sh

@@ -409,6 +409,18 @@ test_expect_success '--set-upstream-to fails on detached HEAD' '
 	git checkout -
 '
 
+test_expect_success '--set-upstream-to fails on a missing dst branch' '
+	test_must_fail git branch --set-upstream-to master does-not-exist
+'
+
+test_expect_success '--set-upstream-to fails on a missing src branch' '
+	test_must_fail git branch --set-upstream-to does-not-exist master
+'
+
+test_expect_success '--set-upstream-to fails on a non-ref' '
+	test_must_fail git branch --set-upstream-to HEAD^{}
+'
+
 test_expect_success 'use --set-upstream-to modify HEAD' '
 	test_config branch.master.remote foo &&
 	test_config branch.master.merge foo &&