Add ".git/config" file parser

Add ".git/config" file parser

This is a first cut at a very simple parser for a git config file.

The format of the file is a simple ini-file like thing, with simple
variable/value pairs. You can (and should) make the variables have a
simple single-level scope, ie a valid file looks something like this:

	#
	# This is the config file, and
	# a '#' or ';' character indicates
	# a comment
	#

	; core variables
	[core]
		; Don't trust file modes
		filemode = false

	; Our diff algorithm
	[diff]
		external = "/usr/local/bin/gnu-diff -u"
		renames = true

which parses into three variables: "core.filemode" is associated with the
string "false", and "diff.external" gets the appropriate quoted value.

Right now we only react to one variable: "core.filemode" is a boolean that
decides if we should care about the 0100 (user-execute) bit of the stat
information. Even that is just a parsing demonstration - this doesn't
actually implement that st_mode compare logic itself.

Different programs can react to different config options, although they
should always fall back to calling "git_default_config()" on any config
option name that they don't recognize.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>