t5520: test --rebase with multiple branches

Since rebasing on top of multiple upstream branches does not make sense,
since 51b2ead (disallow providing multiple upstream branches to rebase,
pull --rebase, 2009-02-18), git-pull explicitly disallowed specifying
multiple branches in the rebase case.

Implement tests to ensure that git-pull fails and prints out the
user-friendly error message in such a case.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5520-pull.sh

@@ -217,6 +217,15 @@ test_expect_success '--rebase' '
 	test "$(git rev-parse HEAD^)" = "$(git rev-parse copy)" &&
 	test new = "$(git show HEAD:file2)"
 '
+
+test_expect_success '--rebase fails with multiple branches' '
+	git reset --hard before-rebase &&
+	test_must_fail git pull --rebase . copy master 2>err &&
+	test "$(git rev-parse HEAD)" = "$(git rev-parse before-rebase)" &&
+	test_i18ngrep "Cannot rebase onto multiple branches" err &&
+	test modified = "$(git show HEAD:file)"
+'
+
 test_expect_success 'pull.rebase' '
 	git reset --hard before-rebase &&
 	test_config pull.rebase true &&