git-gui: Refactor console success/failure handling.

git-gui: Refactor console success/failure handling.

Because I want to be able to run multiple output-producing commands
in a single 'console' window within git-gui I'm refactoring the
console handling routines to require the "after" argument of console_exec.
This should specify a procedure to execute which will receive two args,
the first is the console window handle and the second is the status of
the last command (0 on failure, 1 on success).

A new procedure console_done can be passed to the last console_exec
command to forward perform all cleanup and enable the Close button.
Its status argument is used to update the final status bar on the
bottom of the console window.

This isn't any real logic changing, and no new functionality is in
this patch.

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