git-gui: add configurable tab size to the diff view

For Tk 8.5 the "wordprocessor" mode allows us to get a bit fancy for merge
diffs and intend the tabs by one to compensate for the additional diff
marker at the line start.

The code is heavily based on how gitk handles tabs.

Signed-off-by: Michael Lutz <michi@icosahedron.de>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>