git-proxy updates.

git-proxy updates.

This builds on top of the git-proxy mechanism Paul Collins did,
and updates its configuration mechanism.

 * GIT_PROXY_COMMAND environment variable is used as the
   catch-all fallback, as in the original.  This has not
   changed.

 * Renames proxy configuration variables to core.gitproxy; this
   has become a multi-value variable per list discussion, most
   notably from suggestion by Linus.

	[core]
	;# matches www.kernel.org as well
	gitproxy = netcatter for kernel.org
	gitproxy = netscatter for sample.xz
	gitproxy = none for mydomain.xz
	gitproxy = netcatter-default

   The values are command names, followed by an optional " for "
   and domainname; the first tail-match of the domainname
   determines which proxy command is used.  An entry without "
   for " matches any domain and can be used as the default.

   The command name "none" is special -- it tells the mechanism
   not to use any proxy command and use the native git://
   connection.

Signed-off-by: Junio C Hamano <junkio@cox.net>