verify-tag: update variable name and type

verify-tag: update variable name and type

The run_gpg_verify() function has two variables, size and len.

This may come off as confusing when reading the code. Clarify which one
pertains to the length of the tag headers by renaming len to
payload_size. Additionally, change the type of payload_size to size_t to
match the return type of parse_signature.

Signed-off-by: Santiago Torres <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>