Mercurial [L1 ] is a distributed revision control system with some similarity to git. It is written mostly in Python but can use a Tk viewer hgk to show the branching and patchset information. hgk is closely related to the git viewer gitk.
Mercurial works well on both unix and Windows and has a significantly smaller command set than git which makes it a lot easier to learn to use.
PT has some notes on mirroring the Tcl and Tk CVS repositories via Mercurial [L2 ]