Merge branch 'bc/signed-objects-with-both-hashes'

Signed commits and tags now allow verification of objects, whose two object names (one in SHA-1, the other in SHA-256) are both signed. * bc/signed-objects-with-both-hashes: gpg-interface: remove other signature headers before verifying ref-filter: hoist signature parsing commit: allow parsing arbitrary buffers with headers gpg-interface: improve interface for parsing tags commit: ignore additional signatures when parsing signed commits ref-filter: switch some uses of unsigned long to size_t