Make purges into merges

[orao]

A purge, keep or defunct transaction (on a stream) that clears the entire contents of the stream effectively performs a git rebase action. However, this is not entirely true since Accurev never loses history so the fact that a change was undone remains on the stream's (branch's) history. Hence, it would probably be the best case workaround to make a MergeIntoParent() function that checks if a stream (which isn't timelocked) matches the contents of its parent after a transaction (be it a promote, purge, defunct or otherwise) and if it does we will create a new commit on the parent stream (making sure to update all the children but not snapshots as well) that merges this stream into the parent.

This way we can avoid long ladder chains and squash an effectively empty stream to its parent stream.

[orao]

As well as merging up into the parent we should probably consider reversing the direction of the merges done by MergeIntoChildren() so that it collects all the empty children into an octopus merge (which shouldn't really ever occur if the merging into parent is correct but will be a good backup).

[orao]

Commit d0cd738, which resolved issue #49, is loosely related to the issue discussed here. It attempts to mitigate the graph going hectic by removing ladders in a slightly different way.

[orao]

The best way to address this would be to check all of the child streams that are not timelocked for whether they are the same as this stream at the next promote into this stream and create an octopus merge point with all the child streams that diff the same as additional parents... This would make sure that we eventually collect all children that have been purged of changes.

[orao]

Solving this would also solve issue #60.

[orao]

The strategy of gathering all empty child streams and merging them into our next commit would also resolve #69 and #60.