git-gui: use wordprocessor tab style to ensure tabs work as expected

git-gui: use wordprocessor tab style to ensure tabs work as expected

The Tk text widget tab style is tabular where the first tab will align to
the first tabstop and if that position is left of the current location
then just a single character space is used. With the wordprocessor style
a tab moves the next character position to the next rightmost tabstop
as expected for viewing code.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>