log: do not set up extra_headers for non-email formats

log: do not set up extra_headers for non-email formats

The commit pretty-printer code has an "after_subject" parameter which it
uses to insert extra headers into the email format. In show_log() we set
this by calling log_write_email_headers() if we are using an email
format, but otherwise default the variable to the rev_info.extra_headers
variable.

Since the pretty-printer code will ignore after_subject unless we are
using an email format, this default is pointless. We can just set
after_subject directly, eliminating an extra variable.

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