Merge branch 'cc/fast-import-strip-if-invalid'

"git fast-import" learns "--strip-if-invalid" option to drop invalid cryptographic signature from objects. * cc/fast-import-strip-if-invalid: fast-import: add 'strip-if-invalid' mode to --signed-commits=<mode> commit: refactor verify_commit_buffer() fast-import: refactor finalize_commit_buffer()