log: drop unused "len" from show_tagger()

log: drop unused "len" from show_tagger()

We pass the length of the found "tagger" line to show_tagger(), but it
does not use it; instead, it passes the string to pp_user_info(), which
reads until newline or NUL. This is OK for our purposes because we
always read the object contents into a buffer with an extra NUL (and
indeed, our sole caller already relies on this by using starts_with).

Let's drop the ignored parameter. And while we're touching the caller,
let's use skip_prefix() to avoid a magic number.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>