git-gui: Use git-config now over git-repo-config.

git-gui: Use git-config now over git-repo-config.

Now that core Git has "renamed" git-repo-config to git-config,
we should do the same.  I don't know how long core Git will
keep the repo-config command, and since git-gui's userbase
is so small and almost entirely on some flavor of 1.5.0-rc2
or later, where the rename has already taken place, it should
be OK to rename now.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>