git-gui: set whitespace warnings appropriate to this project

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

.gitattributes

@@ -1,3 +1,4 @@
+*           whitespace=indent-with-non-tab,trailing-space,space-before-tab,tabwidth=4
 *           encoding=US-ASCII
 git-gui.sh  encoding=UTF-8
 /po/*.po    encoding=UTF-8