odb: avoid inflating the full delta to read the header

When we read the header, we want to know the size and type of the
object. We're currently inflating the full delta in order to read the
first few bytes. This can mean hundreds of kB needlessly inflated for
large objects.

Instead use a packfile stream to read just enough so we can read the two
varints in the header and avoid inflating most of the delta.

cmake: include threading libraries in pkg-config

Include any required threading libraries in our `libgit2.pc`.

Introduce `git_signature_from_buffer`

Fix return value of openssl_read (infinite loop)

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