improvements for GIt

After some industrial-strength testing, many thanks to Phil Brooks, I've completely re-worked the Branch Browser so it's faster and more accurate. (If the branches are named, anyway. If they aren't, I can't do a thing with them.)
To improve performance in large repositories, there are two new variables which you can set in your ~/.tkcvs or your site_def:
cvscfg(gitdetail)
If it's false (off) the individual git log call to each file will be suppressed to save time. You won't see the hashtag or committer in the Workdir Browser.
cvscfg(gitmaxhist)
For the branch visualizer. Tells how far back into the history to go. Default is 250 commits.