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: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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