search: Find revisions in hierarchical query

Created by: keegancsmith

We have a separate token to specify refs to search. This allows a more powerful model where we can AND and OR the branches to search. It also leads to a less confusing syntax around the repo names. This commit allows us to inspect a query to find out which references we need to search.

Note: Currently the token is "branch", but this will likely change. As such the language used in this commit is ref since that is the correct language to use for resolving git ref specs.