branch: make create_branch accept a merge base rev

When we ran something like

    $ git checkout -b test master...

it would fail with the message

    fatal: Not a valid object name: 'master...'.

This was caused by the call to `create_branch` where `start_name` is
expected to be a valid rev. However, git-checkout allows the branch to
be a valid _merge base_ rev (i.e. with a "...") so it was possible for
an invalid rev to be passed in.

Make `create_branch` accept a merge base rev so that this case does not
error out.

As a side-effect, teach git-branch how to handle merge base revs as
well.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 13 insertions, 0 deletions

diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
index f1c7023e1a..822381dd9d 100755
--- a/t/t2018-checkout-branch.sh
+++ b/t/t2018-checkout-branch.sh
@@ -66,6 +66,13 @@ test_expect_success 'checkout -b to a new branch, set to HEAD' '
 	do_checkout branch2
 '
 
+test_expect_success 'checkout -b to a merge base' '
+	test_when_finished "
+		git checkout branch1 &&
+		test_might_fail git branch -D branch2" &&
+	git checkout -b branch2 branch1...
+'
+
 test_expect_success 'checkout -b to a new branch, set to an explicit ref' '
 	test_when_finished "
 		git checkout branch1 &&
@@ -126,6 +133,12 @@ test_expect_success 'checkout -B to an existing branch resets branch to HEAD' '
 	do_checkout branch2 "" -B
 '
 
+test_expect_success 'checkout -B to a merge base' '
+	git checkout branch1 &&
+
+	git checkout -B branch2 branch1...
+'
+
 test_expect_success 'checkout -B to an existing branch from detached HEAD resets branch to HEAD' '
 	git checkout $(git rev-parse --verify HEAD) &&