gpg: centralize printing signature buffers

gpg: centralize printing signature buffers

The code to handle printing of signature data from a struct
signature_check is very similar between verify-commit and verify-tag.
Place this in a single function.  verify-tag retains its special case
behavior of printing the tag even when no valid signature is found.

Signed-off-by: brian m. carlson <redacted>
Signed-off-by: Junio C Hamano <redacted>