commit: write commits for both hashes

When we write a commit, we include data that is specific to the hash
algorithm, such as parents and the root tree.  In order to write both a
SHA-1 commit and a SHA-256 version, we need to convert between them.

However, a straightforward conversion isn't necessarily what we want.
When we sign a commit, we sign its data, so if we create a commit for
SHA-256 and then write a SHA-1 version, we'll still have only signed the
SHA-256 data.  While this is valid, it would be better to sign both
forms of data so people using SHA-1 can verify the signatures as well.

Consequently, we don't want to use the standard mapping that occurs when
we write an object.  Instead, let's move most of the writing of the
commit into a separate function which is agnostic of the hash algorithm
and which simply writes into a buffer and specify both versions of the
object ourselves.

We can then call this function twice: once with the SHA-256 contents,
and if SHA-1 is enabled, once with the SHA-1 contents.  If we're signing
the commit, we then sign both versions and append both signatures to
both buffers.  To produce a consistent hash, we always append the
signatures in the order in which Git implemented them: first SHA-1, then
SHA-256.

In order to make this signing code work, we split the commit signing
code into two functions, one which signs the buffer, and one which
appends the signature.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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