ENH: add --commit-label option for 'run' command

[jeremiedbb]

Fixes #370

Add a --commit-label option to be used when running the benchmarks against an existing environment.
It will be used in place of the commit hash (which is None in that case) in order to be able to save the results.

[pv]

Looks ok. Needs test, however.

[jeremiedbb]

It was a little bit less straightforward because the commit hash is truncated to it's 8 first characters for the results file name. However, with provided commit labels I think we want to keep their full length, because the risk of collision is quite high (for instance 'test_label_1' and 'test_label_2' would be cut to 'test_lab').

I chosed to add a prefix for the commit label and check for this prefix when dealing with the result file name. Do you think it's reasonable ?

[jeremiedbb]

I added a test. I wasn't sure where to put it so I made a new test file for the run command. Is there a better place for it ?

[jeremiedbb]

I replace the '-' characters by '_' because the result filename is split by '-' in several places. It should avoid weird behaviors, but may modify the label provided by a user.

[jeremiedbb]

The failing tests seems unrelated and are already failing on master https://travis-ci.org/airspeed-velocity/asv/jobs/506823174

[pv]

I think the main use here is to specify existing commit hashes, when using an existing environment.
Adding support for arbitrary commit labels is more complex, because e.g. the asv web interface assumes commits can be found in the repository, and the use cases for them are not clear.

So I changed this to --set-commit-hash, and removed the label prefix. The prefix also prevents the use case of using asv to run benchmarks but not manage the environment, which I think was the main motivation in #370.