When calling `git_commit_create` with an empty array of `parents` and `parent_count == 0` 
the call will segfault at https://github.com/libgit2/libgit2/blob/master/src/commit.c#L107 
when it's trying to compare `current_id` to a null parent oid. 

This just puts in a check to stop that segfault.

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.

When we moved the logic to handle the first one, wrong loop logic was
kept in place which meant we still finished early. But we now notice it
because we're not reading past the last LF we find.

This was not noticed before as the last field in the tested commit was
multi-line which does not trigger the early break.

Introduce git_commit_extract_signature