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

diff --cc builtin/tag.c
Simple merge

diff --cc commit.c
Simple merge

diff --cc commit.h
Simple merge

diff --cc fmt-merge-msg.c
Simple merge

diff --cc log-tree.c
Simple merge

diff --cc ref-filter.c
Simple merge

diff --cc t/t7004-tag.sh
Simple merge

diff --cc t/t7510-signed-commit.sh
Simple merge