Merge branch 'strg-t-in-git-gui'

This does not (yet) resolve the problem that Strg+T with multiple
selected lines fails to (un)stage them all, but it addresses one of
the reported Ctrl+T issues.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

git-gui/git-gui.sh

@@ -2516,6 +2516,10 @@ proc toggle_or_diff {mode w args} {
 		if {$last_clicked ne {}} {
 			set lno [lindex $last_clicked 1]
 		} else {
+			if {[llength $file_lists($w)] == 0} {
+				set last_clicked {}
+				return
+			}
 			set lno [expr {int([lindex [$w tag ranges in_diff] 0])}]
 		}
 		if {$mode eq "toggle"} {