git-gui: use different icon for new and modified files in the index

This allows to quickly differentiate between new and modified files
in the index without selecting the file and looking at the diff.

Signed-off-by: Peter Oberndorfer <kumbayo84@arcor.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Author: kumbayo

git-gui.sh

@@ -1904,7 +1904,7 @@ set ui_index .vpane.files.index.list
 set ui_workdir .vpane.files.workdir.list
 
 set all_icons(_$ui_index)   file_plain
-set all_icons(A$ui_index)   file_fulltick
+set all_icons(A$ui_index)   file_plain
 set all_icons(M$ui_index)   file_fulltick
 set all_icons(D$ui_index)   file_removed
 set all_icons(U$ui_index)   file_merge