return a link from knotserver to quickly compare new branches against default branch #223

when a new branch is pushed to a git repository on a knot, return a /compare link so users can open a PR with 2 clicks. the logic is roughly:

• detect new branches (this can be detected in the hooks as a ref where the hash moves from 000... to someHash
• for every new ref, produce a link that is of the form TANGLED_APPVIEW/did/repo/compare/defaultBranch...newRef
• send these refs back to the user in the ssh response. see MOTD for an example on how this is done.