log: --show-signature

This teaches the "log" family of commands to pass the GPG signature in the
commit objects to "gpg --verify" via the verify_signed_buffer() interface
used to verify signed tag objects. E.g.

    $ git show --show-signature -s HEAD

shows GPG output in the header part of the output.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -218,4 +218,6 @@ struct merge_remote_desc {
  */
 struct commit *get_merge_parent(const char *name);
 
+extern int parse_signed_commit(const unsigned char *sha1,
+			       struct strbuf *message, struct strbuf *signature);
 #endif /* COMMIT_H */