Re: n-heads and patch dependency chains

First, hydras or n-head was invented to avoid capping and recapping, and just advance it as a normal head (and to remember what are subprojects/patch dependency chains/whatever to choose).

Second, we could generalize those extra commit references in commit structure (be they "bind", "prior" or "previous", or "depends-on") and have commit/merge pluggable helper manage them. And merge strategy may make use of them.

Third, would using *directory* with for a N-HEAD (containing all the subheads, subprojects, chains, branches, fibers, whatever) instead of an ordinary file for HEAD be a good idea? For hydra if we want it to be easily interweaved with ordinary commit I think we would also need the link for bottom, hydra shoulder, hydra tail i.e. common commit being starting point for all the chains, or subprojects (for subprojects it can be empty tree commit).