git-gui: Refactor to use our git proc more often

git-gui: Refactor to use our git proc more often

Whenever we want to execute a git subcommand from the plumbing
layer (and on rare occasion, the more porcelain-ish layer) we
tend to use our proc wrapper, just to make the code slightly
cleaner at the call sites.  I wasn't doing that in a couple of
places, so this is a simple cleanup to correct that.

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