git-gui: Add a Tools menu for arbitrary commands.

git-gui: Add a Tools menu for arbitrary commands.

Due to the emphasis on scriptability in the git
design, it is impossible to provide 100% complete
GUI. Currently unaccounted areas include git-svn
and other source control system interfaces, TopGit,
all custom scripts.

This problem can be mitigated by providing basic
customization capabilities in Git Gui. This commit
adds a new Tools menu, which can be configured
to contain items invoking arbitrary shell commands.

The interface is powerful enough to allow calling
both batch text programs like git-svn, and GUI editors.
To support the latter use, the commands have access
to the name of the currently selected file through
the environment.

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