git-gui: Implement automatic rescan after Tool execution.

git-gui: Implement automatic rescan after Tool execution.

The Tools menu is generally intended for commands that
affect the working directory or repository state. Thus,
the user would usually want to initiate rescan after
execution of a tool. This commit implements it.

In case somebody would want to avoid rescanning after
certain tools, it also adds an option that controls it,
although it is not made available through the Add dialog.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>