git-gui: Add a --trace command line option

git-gui: Add a --trace command line option

Often new Git users want to know what commands git-gui uses to make
changes, so they can learn the command line interface by mimicking
what git-gui does in response to GUI actions.  Showing the direct
commands being executed is easy enough to implement but this is of
little value to end-users because git-gui frequently directly calls
plumbing, not porcelain.

Since the code is already written and tested, its fairly harmless
to include.  It may not help a new end-user, but it can help with
debugging git-gui or reverse-engineering its logic to further make
changes to it or implement another GUI for Git.

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