This refactors the implementation of gathering/iterating changes
from the post-receive hook to be a bit more readable (removed
the need for calling `enum_for(:changes_refs)` in different
places).

This is done with the help of the new `Gitlab::Git::Changes`
collection object which will be used while parsing the changes.

Also changed the signature of BranchPushService and TagPushService
to accept a single `change` param instead of having oldrev, newrev
and ref params.

This also prepares the `PostReceive` worker to be able to process
branches and tags separately so we'll be able to aggregate events
by branches or tags.