Multi-tier repository structure

Thanks, Richard. I definitely intend to integrate multi-tier directories before cutting a first beta release. I will review and test your solution for the next alpha.

I've now implemented initial internal support for multi-tier repository structures in the 6.x-1.0-alpha3 release. I proceeded from your patch but expanded on it as follows:

There is not yet any UI-level support for creating multi-tier repositories, but there are two Drupal system variables (named bitcache_depth and bitcache_width) that you can set manually in order to get support for any conceivable multi-tier repository configuration. In the next release, I will make it possible to configure these on a repository-by-repository basis via the administrative interface. (I plan to keep the zero-tier structure as the default as most people using Bitcache on Drupal are likely to have modest needs, and this also avoids the need for any potentially risky data migration updates.)

By default, both variables are set to zero which results in the familiar zero-tier, or "flat", repository. To get the exact two-tier structure that you wanted, simply set bitcache_depth to 2 and bitcache_width to 1. For the time being, you can modify these at the top of boost.module or in your variable MySQL table as follows:

To demonstrate the flexibility these variables provide for configuring repository structures, consider a case where you wanted something like the object store structure that Git uses internally; to achieve that, you would just swap the two variables, setting bitcache_depth to 1 and bitcache_width to 2; this would yield up to 16x16 (= 256) two-character top-level directories such as e.g. /ac/acc34176f7f83d7bc0a1a91250dd4506a18ff8dd. (In fact, this would in principle allow you to plug in an existing Git object repository into Bitcache.)

Hopefully this information suffices to make use of this functionality for the time being, should you need it. I'll write more documentation in time for the next release...