Feature request: "revision" field in rockspecs

Nobody has claimed this yet.

example:

```
source = {
url = "git://my/repo",
revision = "86d59e66d0c12656ed34d4f4a9bab06ac7775f77"
}
```

In this context revision means an atomic reference to a repo's state, e.g. commit hashes in git and changeset IDs in Mercurial.

This can be useful for when you don't directly control a source repo to add tags to it, or if you want more specific rockspecs than you want tags. It can also be used as verification for tags, the same way source.md5 is used.

Implementation-wise, it doesn't look like git lets you clone a specific commit, so instead you need to pick the closest branch/tag and then `git checkout` from there. Mercurial's `--rev` flag also does changesets in addition to branches/tags so there it's just a matter of also passing the changeset id in.

Start by tracing how rockspec source fields are parsed and how Git and Mercurial sources are fetched. Check the existing branch and tag handling, including Git checkout and Mercurial revision arguments. Done means a revision can be declared in a rockspec for both backends and is used to retrieve the requested repository state, with coverage for the new behavior.

Written by the indexing model from the issue text.