git-gui: Cache all repo-config data in an array.

git-gui: Cache all repo-config data in an array.

We're likely going to need key/value pairs from the repo-config beyond
just remote.*.url, so cache them all up front into a Tcl array where we
have fast access to them without needing to refork a repo-config --list
process.

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