Teach Git how to parse standard power of 2 suffixes.

Teach Git how to parse standard power of 2 suffixes.

Sometimes its necessary to supply a value as a power of two in a
configuration parameter.  In this case the user may want to use the
standard suffixes such as K, M, or G to indicate that the numerical
value should be multiplied by a constant base before being used.

Shell scripts/etc. can also benefit from this automatic option
parsing with `git repo-config --int`.

[jc: with a couple of test and a slight input tightening]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>