t3432: test rebase fast-forward behavior

When rebase is run on a branch that can be fast-forwarded, this should
automatically be done. Create test to ensure this behavior happens.

There are some cases that currently don't pass. The first case is where
a feature and master have diverged, running
"git rebase master... master" causes a full rebase to happen even though
a fast-forward should happen.

The second case is when we are doing "git rebase --fork-point" and a
fork-point commit is found. Once again, a full rebase happens even
though a fast-forward should happen.

Mark these cases as failure so we can fix it later.

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

t/t3432-rebase-fast-forward.sh

@@ -0,0 +1,72 @@
+#!/bin/sh
+#
+# Copyright (c) 2019 Denton Liu
+#
+
+test_description='ensure rebase fast-forwards commits when possible'
+
+. ./test-lib.sh
+
+test_expect_success setup '
+	test_commit A &&
+	test_commit B &&
+	test_commit C &&
+	test_commit D &&
+	git checkout -t -b side
+'
+
+test_rebase_same_head () {
+	status="$1" &&
+	shift &&
+	test_expect_$status "git rebase $* with $changes is no-op" "
+		oldhead=\$(git rev-parse HEAD) &&
+		test_when_finished 'git reset --hard \$oldhead' &&
+		git rebase $* &&
+		newhead=\$(git rev-parse HEAD) &&
+		test_cmp_rev \$oldhead \$newhead
+	"
+}
+
+changes='no changes'
+test_rebase_same_head success
+test_rebase_same_head success master
+test_rebase_same_head success --onto B B
+test_rebase_same_head success --onto B... B
+test_rebase_same_head success --onto master... master
+test_rebase_same_head success --no-fork-point
+test_rebase_same_head success --fork-point master
+test_rebase_same_head failure --fork-point --onto B B
+test_rebase_same_head failure --fork-point --onto B... B
+test_rebase_same_head success --fork-point --onto master... master
+
+test_expect_success 'add work to side' '
+	test_commit E
+'
+
+changes='our changes'
+test_rebase_same_head success
+test_rebase_same_head success master
+test_rebase_same_head success --onto B B
+test_rebase_same_head success --onto B... B
+test_rebase_same_head success --onto master... master
+test_rebase_same_head success --no-fork-point
+test_rebase_same_head success --fork-point master
+test_rebase_same_head failure --fork-point --onto B B
+test_rebase_same_head failure --fork-point --onto B... B
+test_rebase_same_head success --fork-point --onto master... master
+
+test_expect_success 'add work to upstream' '
+	git checkout master &&
+	test_commit F &&
+	git checkout side
+'
+
+changes='our and their changes'
+test_rebase_same_head success --onto B B
+test_rebase_same_head success --onto B... B
+test_rebase_same_head failure --onto master... master
+test_rebase_same_head failure --fork-point --onto B B
+test_rebase_same_head failure --fork-point --onto B... B
+test_rebase_same_head failure --fork-point --onto master... master
+
+test_done