git-gui: new config to control staging of untracked files

The default is the current "ask".

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>
diff --git a/git-gui.sh b/git-gui.sh
index e5dd8bc..7eeec52 100755
--- a/git-gui.sh
+++ b/git-gui.sh
@@ -859,6 +859,7 @@
 	{fontui   font_ui   {mc "Main Font"}}
 	{fontdiff font_diff {mc "Diff/Console Font"}}
 }
+set default_config(gui.stageuntracked) ask
 
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