doc: gitk: add the upstream repo location

Match the 'git gui' information regarding the graphical browser
and its upstream location.

Signed-off-by: Philip Oakley <philipoakley@iee.org>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Philip Oakley

Documentation/gitk.txt

@@ -178,9 +178,15 @@ used by default. If '$XDG_CONFIG_HOME' is not set it defaults to
 History
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 Gitk was the first graphical repository browser. It's written in
-tcl/tk and started off in a separate repository but was later merged
-into the main Git repository.
+tcl/tk.
 
+'gitk' is actually maintained as an independent project, but stable
+versions are distributed as part of the Git suite for the convenience
+of end users.
+
+gitk-git/ comes from Paul Mackerras's gitk project:
+
+	git://ozlabs.org/~paulus/gitk
 
 SEE ALSO
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