commit: add tests of commit races

Committing involves the following steps:

1. Determine the current value of HEAD (if any).
2. Create the new commit object.
3. Update HEAD.

Please note that step 2 can take arbitrarily long, because it might
involve the user editing a commit message.

If a second process sneaks in a commit during step 2, then the first
commit process should fail. This is usually done correctly, because
step 3 verifies that HEAD still points at the same commit that it
pointed to during step 1.

However, if there is a race when creating an *orphan* commit, then the
test in step 3 is skipped.

Add tests for proper handling of such races. One of the new tests
fails. It will be fixed in a moment.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t7516-commit-races.sh

+#!/bin/sh
+
+test_description='git commit races'
+. ./test-lib.sh
+
+test_expect_failure 'race to create orphan commit' '
+	write_script hare-editor <<-\EOF &&
+	git commit --allow-empty -m hare
+	EOF
+	test_must_fail env EDITOR=./hare-editor git commit --allow-empty -m tortoise -e &&
+	git show -s --pretty=format:%s >subject &&
+	grep hare subject &&
+	test -z "$(git show -s --pretty=format:%P)"
+'
+
+test_expect_success 'race to create non-orphan commit' '
+	write_script airplane-editor <<-\EOF &&
+	git commit --allow-empty -m airplane
+	EOF
+	git checkout --orphan branch &&
+	git commit --allow-empty -m base &&
+	git rev-parse HEAD >base &&
+	test_must_fail env EDITOR=./airplane-editor git commit --allow-empty -m ship -e &&
+	git show -s --pretty=format:%s >subject &&
+	grep airplane subject &&
+	git rev-parse HEAD^ >parent &&
+	test_cmp base parent
+'
+
+test_done