git-gui: fix deleting from the context menu with empty selection

An "Application Error" was raised when trying to delete text from the
commit message field when no text was selected.

Signed-off-by: Markus Heidelberg <markus.heidelberg@web.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -2944,7 +2944,7 @@ $ctxm add command \
 	-command {tk_textPaste $ui_comm}
 $ctxm add command \
 	-label [mc Delete] \
-	-command {$ui_comm delete sel.first sel.last}
+	-command {catch {$ui_comm delete sel.first sel.last}}
 $ctxm add separator
 $ctxm add command \
 	-label [mc "Select All"] \