Teach bash about git-repo-config.

Teach bash about git-repo-config.

This is a really ugly completion script for git-repo-config, but it has
some nice properties.  I've added all of the documented configuration
parameters from Documentation/config.txt to the script, allowing the
user to complete any standard configuration parameter name.

We also have some intelligence for the remote.*.* and branch.*.* keys
by completing not only the key name (e.g. remote.origin) but also the
values (e.g. remote.*.fetch completes to the branches available on the
corresponding remote).

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