Implement branch.squash

[ChrisPenner]

Overview

Implements branch.squash by request from @ceedubs and @stew, which should allow them to creating small sync files quickly which will be nice for speeding up some of their docker builds.

Note that there still doesn't appear to be a clone.squashed, which would probably be nice for testing trunk builds 🤔

Implementation notes

• Adds new branch.squash command.
  • branch.squash src dest squashes src into dest. Where src and dest can be project/branch /branch etc.

Interesting/controversial decisions

I started by allowing it to accept a causal hash, path, etc. but the input parsing drove me nuts so I backstepped to something simpler.

Test coverage

Transcripts

[ceedubs]

Build failures aside this seems good to me!

[ChrisPenner]

I've finally wrangled myself into passing transcripts haha @aryairani

[aryairani]

We'll have to re-run transcripts after merging #5705, so I'll hold off until after that.