t2024: Add tests verifying current DWIM behavior of 'git checkout <branch>'

The DWIM mode of checkout allows you to run "git checkout foo" when there is
no existing local ref or path called "foo", and there is exactly one remote
with a remote-tracking branch called "foo". Git will then automatically
create a new local branch called "foo" using the remote-tracking "foo" as
its starting point and configured upstream.

Improved-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t2024-checkout-dwim.sh

@@ -0,0 +1,99 @@
+#!/bin/sh
+
+test_description='checkout <branch>
+
+Ensures that checkout on an unborn branch does what the user expects'
+
+. ./test-lib.sh
+
+# Is the current branch "refs/heads/$1"?
+test_branch () {
+	printf "%s\n" "refs/heads/$1" >expect.HEAD &&
+	git symbolic-ref HEAD >actual.HEAD &&
+	test_cmp expect.HEAD actual.HEAD
+}
+
+# Is branch "refs/heads/$1" set to pull from "$2/$3"?
+test_branch_upstream () {
+	printf "%s\n" "$2" "refs/heads/$3" >expect.upstream &&
+	{
+		git config "branch.$1.remote" &&
+		git config "branch.$1.merge"
+	} >actual.upstream &&
+	test_cmp expect.upstream actual.upstream
+}
+
+test_expect_success 'setup' '
+	git init repo_a &&
+	(
+		cd repo_a &&
+		test_commit a_master &&
+		git checkout -b foo &&
+		test_commit a_foo &&
+		git checkout -b bar &&
+		test_commit a_bar
+	) &&
+	git init repo_b &&
+	(
+		cd repo_b &&
+		test_commit b_master &&
+		git checkout -b foo &&
+		test_commit b_foo &&
+		git checkout -b baz &&
+		test_commit b_baz
+	) &&
+	git remote add repo_a repo_a &&
+	git remote add repo_b repo_b &&
+	git config remote.repo_b.fetch \
+		"+refs/heads/*:refs/remotes/other_b/*" &&
+	git fetch --all
+'
+
+test_expect_success 'checkout of non-existing branch fails' '
+	git checkout -B master &&
+	test_might_fail git branch -D xyzzy &&
+
+	test_must_fail git checkout xyzzy &&
+	test_must_fail git rev-parse --verify refs/heads/xyzzy &&
+	test_branch master
+'
+
+test_expect_success 'checkout of branch from multiple remotes fails' '
+	git checkout -B master &&
+	test_might_fail git branch -D foo &&
+
+	test_must_fail git checkout foo &&
+	test_must_fail git rev-parse --verify refs/heads/foo &&
+	test_branch master
+'
+
+test_expect_success 'checkout of branch from a single remote succeeds #1' '
+	git checkout -B master &&
+	test_might_fail git branch -D bar &&
+
+	git checkout bar &&
+	test_branch bar &&
+	test_cmp_rev remotes/repo_a/bar HEAD &&
+	test_branch_upstream bar repo_a bar
+'
+
+test_expect_success 'checkout of branch from a single remote succeeds #2' '
+	git checkout -B master &&
+	test_might_fail git branch -D baz &&
+
+	git checkout baz &&
+	test_branch baz &&
+	test_cmp_rev remotes/other_b/baz HEAD &&
+	test_branch_upstream baz repo_b baz
+'
+
+test_expect_success '--no-guess suppresses branch auto-vivification' '
+	git checkout -B master &&
+	test_might_fail git branch -D bar &&
+
+	test_must_fail git checkout --no-guess bar &&
+	test_must_fail git rev-parse --verify refs/heads/bar &&
+	test_branch master
+'
+
+test_done