git-gui: Refactor 'exec git subcmd' idiom.

git-gui: Refactor 'exec git subcmd' idiom.

As we frequently need to execute a Git subcommand and obtain
its returned output we are making heavy use of [exec git foo]
to run foo.  As I'm concerned about possibly needing to carry
environment data through a shell on Cygwin for at least some
subcommands, I'm migrating all current calls to a new git
proc.  This actually makes the code look cleaner too, as
we aren't saying 'exec git' everywhere.

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