Add VCS revision as `scm_revision` in tab

- record the source control commit hash in
  `INSTALL_RECEIPT.json` when a formula is built
  from a VCS download strategy (Git, SVN, Hg,
  Bazaar, Fossil)
- expose `source_revision` on download strategies
  so `Tab.create` can capture it from the active
  spec's downloader when the cached checkout exists
- propagate `scm_revision` through `to_bottle_hash`
  for bottle JSON metadata