Merge branch 'jt/fast-import-sign-again' into next

"git fast-import" learned to optionally replace signature on
commits whose signatures get invalidated due to replaying by
signing afresh.

* jt/fast-import-sign-again:
  fast-import: add mode to sign commits with invalid signatures
  gpg-interface: allow sign_buffer() to use default signing key
  commit: remove unused forward declaration

diff --cc builtin/fast-import.c
Simple merge

diff --cc builtin/tag.c
Simple merge

diff --cc t/t9305-fast-import-signatures.sh
Simple merge