proof-of-concept search select for batch changes

Created by: rvantonder

Disclaimer: this is completely untested / unvalidated, but I'm putting up a proof-of-concept rather than writing a slack thread about it

Batch changes will benefit from using the new select: parameter for filtering repos/files. The main reasons are:

• select: automatically deduplicates repos and files (no need to send everything over the wire for it to get deduplicated here)
• For select:repo specifically, we added an optimization that will stop searching repos if one result is found (we don't continue searching for count:99999 matches). This "should" be faster.
• select: has a more natural behavior for describing searches for batch changes compared to type: ( select: only affects result types, type: may interpret inputs in various ways).

You'll have to try this out, maybe like a beta? I'm not sure how this part is tested, but it should be a worthwhile update. Please let me know if you do try it out and something seems wrong.

Feel free to close this PR whenever, it's meant for visibility and discussion.