There can be a significant difference between the system where we created the
buffer (if at all) and when the caller provides us with the contents of a
commit.

Verify that the commit we are being asked to create references objects which do
exist in the target repository.

DRY commit parsing

This allows us to pick which data from a commit we're interested in.
This will be used by the revwalk code, which is only interested in
parents' and committer data.

If provided with a null signature, skip adding the signature header and create the commit anyway.

Move to the `git_error` name in the internal API for error-related
functions.

Use the new object_type enumeration names within the codebase.

While commit objects usually should have only one author field, our commit
parser actually handles the case where a commit has multiple author fields
because some tools that exist in the wild actually write them. Detection of
those additional author fields is done by using a simple `git__prefixcmp`,
checking whether the current line starts with the string "author ". In case
where we are handed a non-NUL-terminated string that ends directly after the
space, though, we may have an out-of-bounds read of one byte when trying to
compare the expected final NUL byte.

Fix the issue by using `git__prefixncmp` instead of `git_prefixcmp`.
Unfortunately, a test cannot be easily written to catch this case. While we
could test the last error message and verify that it didn't in fact fail parsing
a signature (because that would indicate that it has in fact tried to parse the
additional "author " field, which it shouldn't be able to detect in the first
place), this doesn't work as the next line needs to be the "committer" field,
which would error out with the same error message even if we hadn't done an
out-of-bounds read.

As objects read from the object database are always NUL terminated, this issue
cannot be triggered in normal code and thus it's not security critical.