Implement support for revsets

[arxanas]

See discussion at #157.

[claytonrcarter]

This probably won't be helpful, but FWIW I hacked around a little bit last night was able to get this working w/ the following changes in dag.rs:

+use gitrevset::SetExt;

pub fn resolve_commits<'repo>(
    effects: &Effects,
    repo: &'repo Repo,
    dag: &mut Dag,
    hashes: Vec<String>,
) -> eyre::Result<ResolveCommitsResult<'repo>> {
    let mut commits = Vec::new();
+
+    // FIXME can we share the already opened git2::Repository from repo.inner?
+    let git_repo = git2::Repository::open(repo.get_path())?;
+    let revset_repo = gitrevset::Repo::open_from_repo(Box::new(git_repo))?;
+
    for hash in hashes {
-       let commit = match repo.revparse_single_commit(&hash)? {
+       let set = revset_repo.anyrevs(hash.as_str())?;
+
+       for oid in set.to_oids()? {
+           // FIXME yuck: oid -> to_string -> as_str ???
+           let commit = match repo.revparse_single_commit(oid?.to_string().as_str())? {
            let commit = match repo.revparse_single_commit(&hash)? {
                Some(commit) => commit,
                None => return Ok(ResolveCommitsResult::CommitNotFound { commit: hash }),
            };
            commits.push(commit)
        }
+    }

    let commit_oids = commits.iter().map(|commit| commit.get_oid()).collect_vec();
    dag.sync_from_oids(
        effects,
        repo,
        CommitSet::empty(),
        CommitSet::from_iter(commit_oids.into_iter().map(CommitVertex::from).map(Ok)),
    )?;
    Ok(ResolveCommitsResult::Ok { commits })
}

Not pretty and theres room for much improvement, but it was fun to play with and this was enough to get something like git hide 'desc("foo")' to actually work.