repo: factor out field printing to dedicated function

repo: factor out field printing to dedicated function

Move the field printing in git-repo-info to a new function called
`print_field`, allowing it to be called by functions other than
`print_fields`.

Also change its use of quote_c_style() helper to output directly to
the standard output stream, instead of taking a result in a strbuf
and then printing it outselves.

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