config: use unsigned_mult_overflows to check for overflows

config: use unsigned_mult_overflows to check for overflows

parse_unit_factor() checks if a K, M or G is present after a number and
multiplies it by 2^10, 2^20 or 2^30, respectively.  One of its callers
checks if the result is smaller than the number alone to detect
overflows.  The other one passes 1 as the number and does multiplication
and overflow check itself in a similar manner.

This works, but is inconsistent, and it would break if we added support
for a bigger unit factor.  E.g. 16777217T is 2^64 + 2^40, i.e. too big
for a 64-bit number.  Modulo 2^64 we get 2^40 == 1TB, which is bigger
than the raw number 16777217 == 2^24 + 1, so the overflow would go
undetected by that method.

Let both callers pass 1 and handle overflow check and multiplication
themselves.  Do the check before the multiplication, using
unsigned_mult_overflows, which is simpler and can deal with larger unit
factors.

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