config: simplify parsing of unit factors

config: simplify parsing of unit factors

Just return the value of the factor or zero for unrecognized strings
instead of using an output reference and a separate return value to
indicate success.  This is shorter and simpler.

It basically reverts that function to before c8deb5a146 ("Improve error
messages when int/long cannot be parsed from config", 2007-12-25), while
keeping the better messages, so restore its old name, get_unit_factor(),
as well.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>