Simplify map_user(), mostly to avoid copies of string buffers. It
also simplifies caller functions.

map_user() directly receive pointers and length from the commit buffer
as mail and name. If mapping of the user and mail can be done, the
pointer is updated to a new location. Lengths are also updated if
necessary.

The caller of map_user() can then copy the new email and name if
necessary.

Signed-off-by: Antoine Pelisse <apelisse@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>