git-gui: Correctly handle spaces in filepaths.

git-gui: Correctly handle spaces in filepaths.

Anytime are about to open a pipe on what may be user data we need to
make sure the value is escaped correctly into a Tcl list, so that the
executed subprocess will receive the right arguments.  For the most
part we were already doing this correctly, but a handful of locations
did not.

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