git-gui: ensure the file in the diff pane is in the list of selected files

It is very confusing that the file which diff is displayed is marked as
selected, but it is not in fact selected (that means the array of selected
files does not include the file in question).

Fixing this also improves the use of $FILENAMES in custom defined tools: one
does not have to click the file in the list to make it selected.
Signed-off-by: NAlex Riesen <alexander.riesen@cetitec.com>
Signed-off-by: NPat Thoyts <patthoyts@users.sourceforge.net>

lib/diff.tcl

@@ -127,6 +127,9 @@ proc show_diff {path w {lno {}} {scroll_pos {}} {callback {}}} {
 	} else {
 		start_show_diff $cont_info
 	}
+
+	global current_diff_path selected_paths
+	set selected_paths($current_diff_path) 1
 }
 
 proc show_unmerged_diff {cont_info} {