API should support fetching builds by VCS revision / hash.
Discussed here: https://twitter.com/FelicianoTech/status/1027242152965877760
During some non-CircleCI deployment processes, one part of the configuration might be the specific git revision of the code to be deployed (e.g. to get an old build artifact to be able to deploy an old build instantly). To do this with the API, currently you have to look through all the builds in the project, paging through them 100 items at a time and making a new API call each time until you find the build you need, to then fetch the artifact URL.
I propose an addition of a new query parameter to `GET: /project/:vcs-type/:username/:project`, similar to `?filter=blah`. We could have `?revision=<hash>`, which would filter the list of builds to only those matching the given VCS revision. This would make finding builds by hash a one-step process.