Add batched merge (rollup) feature

[barosl]

Currently it works as follows:

1. Mark the commits to be merged together as rollup. (e.g. r+ rollup)
   These commits will have an implicit priority of -1 to postpone the individual merge.
2. If one of the marked commits reaches the top of the queue, all the marked commits will be merged together and tested.
3. While merging commits, those commits that fail to merge are ignored.
4. You can prioritize the rollup by setting p=.

Typical usage:

1. We have three commits to be tested together.
2. Set r+ rollup to the first commit.
3. Set r+ rollup to the second commit.
4. Set r+ rollup p=10 to the last commit, which will trigger the rollup process soon.

Fixes #34.

[barosl]

You can see the working sample from here: barosltestest/test#49

[barosl]

By defaults, bors checks the freshness of the merge commit before fast-forwarding it to master. "Fresh" in this case means that the two direct parents of the merge commit is exactly the HEADs of the master branch and the test branch ("auto"), meaning no other commits are pushed to master while testing the candidate.

As a batched merge commit does not have master as a direct parent, the freshness check always fails. So, this PR simply bypasses the check if the pending job is a rollup. This workaround is not elegant, but in reality it wouldn't make a serious problem because the fast-forward itself ensures the freshness.

[Gankra]

In order for the Rust project to take advantage of this, it would need to be based on the rust branch, which has diverged from master in incompatible ways.