doc: update links to current pages

It's somewhat traditional to respect sites' self-identification.

[j6t: cherry-picked from 65175d9]

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>

Author: jsoref

git-gui.sh

@@ -2367,7 +2367,7 @@ proc do_quit {{rc {1}}} {
 	set ret_code $rc
 
 	# Briefly enable send again, working around Tk bug
-	# http://sourceforge.net/tracker/?func=detail&atid=112997&aid=1821174&group_id=12997
+	# https://sourceforge.net/p/tktoolkit/bugs/2343/
 	tk appname [appname]
 
 	destroy .