Enhancement: show or filter descendants

[gcardwel]

Sometimes when reviewing branch history, especially one that is very "branchy", It's difficult even with the branch colorization to see what later commits are descendants of an earlier commit or branch.

With gitk, you can easily show this with:

gitk --all --ancestry-path CHANGESET-SHA..

and get a visual of what comes from that CHANGESET. A selection in the "view" menu that filters would be very useful for minimizing the clutter, and could work similarly to the existing "filter by active branch" option.