Split up default "core" config parsing into helper routine

Split up default "core" config parsing into helper routine

It makes the code a bit easier to read, and in theory a bit faster too
(no need to compare all the different "core.*" strings against non-core
config options).

The config system really should get something of a complete overhaul,
but in the absense of that, this at least improves on it a tiny bit.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>