`test_commit_commit__create_initial_commit_parent_not_current` was not correctly 
testing that `HEAD` was not changed. Now we grab the oid that it was pointing to
before the call to `git_commit_create` and the oid that it's pointing to afterwards
and compare those.

Allow users to construct a signature from the type of signature
lines that actually appear in commits.

commit: add function to attach a signature to a commit

The function to extract signatures suffers from a similar bug to the
header field finding one by having an unecessary line feed check as a
break condition of its loop.

Fix that and add a test for this single-line signature situation.

In combination with the function which creates a commit into a buffer,
this allows us to more easily create signed commits.

Sometimes you want to create a commit but not write it out to the
objectdb immediately. For these cases, provide a new function to
retrieve the buffer instead of having to go through the db.

When `GIT_OPT_ENABLE_STRICT_OBJECT_CREATION` is turned on, validate
the tree and parent ids given to commit creation functions.

We should be checking whether the object we're looking up is a commit,
and we should let the caller know whether the not-found return code
comes from a bad object type or just a missing signature.