Fetch job by ref instead of commit

[tarajdactyl]

Currently art resolves ref to its commit hash and then queries jobs using the sha field.

This can result in incorrect results. In particular, consider a policy of only retaining artifacts for tags and latest:

job id	commit hash	ref	artifacts
2	abcdefdeadbeef	master	(deleted)
1	abcdefdeadbeef	v1.0.0	artifacts.zip

in this case, fetching aritfacts for ref: v1.0.0 results in resolving v1.0.0 to abcdefdeadbeef and then getting the last successful job, (2), whose artifacts have been deleted.

If, on the other hand, the job was queried by ref, it would have returned 1.