Read configuration also from $HOME/.gitconfig

Read configuration also from $HOME/.gitconfig

This patch is based on Pasky's, with three notable differences:

- I did not yet update the documentation
- I named it .gitconfig, not .gitrc
- git-repo-config does not barf when a unique key is overridden locally

The last means that if you have something like

	[alias]
		l = log --stat -M

in ~/.gitconfig, and

	[alias]
		l = log --stat -M next..

in $GIT_DIR/config, then

	git-repo-config alias.l

returns only one value, namely the value from $GIT_DIR/config.

If you set the environment variable GIT_CONFIG, $HOME/.gitconfig is not
read, and neither $GIT_DIR/config, but $GIT_CONFIG instead.

If you set GIT_CONFIG_LOCAL instead, it is interpreted instead of
$GIT_DIR/config, but $HOME/.gitconfig is still read.

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>