tkdiff Code

same directory as the first file when looking for the second one. Will
still have to do something about when it's started with a double click.
* Detect PVCS by environment variable (patch 1839361 by nafmo)
* Update BitKeeper support (patch 3053551 by wscott)
* Mercurial support (patch 1867700 by damonmc)
* Rudimentary Git support (patch 1836293 by cecilh3)
* add help menu items to report versions of wish and diff