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.

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

This returns the GPG signature for a commit and its contents without the
signature block, allowing for the verification of the commit's
signature.

We were searching only past the first header field, which meant we were
unable to find e.g. `tree` which is the first field.

While here, make sure to set an error message in case we cannot find the
field.

This allows the user to look up fields which we don't parse in libgit2,
and allows them to access gpgsig or mergetag fields if they wish to
check the signature.

Some tools create multiple author fields. git is rather lax when parsing
them, although fsck does complain about them. This means that they exist
in the wild.

As it's not too taxing to check for them, and there shouldn't be a
noticeable slowdown when dealing with correct commits, add logic to skip
over these extra fields when parsing the commit.