Refactor commit messge handling.

- Move handle_info into main so it is called once
  after everything has been parsed.  This allows the removal
  of a static variable and removes two duplicate calls.

- Move parsing of inbody headers into handle_commit.
  This means we parse the in-body headers after we have decoded
  the character set, and it removes code duplication between
  handle_multipart_one_part and handle_body.

- Change the flag indicating that we have seen an in body
  prefix header into another bit in seen.
  This is a little more general and allows the possibility of parsing
  in body headers after the body message has begun.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>