repo: add a default output format to enum output_format

repo: add a default output format to enum output_format

Add a `FORMAT_DEFAULT` value to `enum output_format`. Change the initial
value of `format` to `FORMAT_DEFAULT` in cmd_repo_info, indicating that
the initial value hasn't been changed. Also map the string "default" to
this new value in `parse_format_cb`, allowing future patches to add
support to --format=default.

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>