Add functions for parsing integers with size suffixes

Add functions for parsing integers with size suffixes

Split out the nnn{k,m,g} parsing code from git_config_int into
git_parse_long, so command-line parameters can enjoy the same
functionality.  Also add get_parse_ulong for unsigned values.

Make git_config_int use git_parse_long, and add get_config_ulong
as well.

Signed-off-by: Brian Downing <bdowning@lavos.net>
Acked-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>