filter-branch: introduce convenience function "skip_commit"
With this function, a commit filter can leave out unwanted commits
(such as temporary commits). It does _not_ undo the changeset
corresponding to that commit, but it _skips_ the revision. IOW
no tree object is changed by this.
If you like to commit early and often, but want to filter out all
intermediate commits, marked by "@@@" in the commit message, you can
now do this with