R1:4928c34d00c2

Allow "bin/bulk export" to merge multiple queries and accept more flexible flags

Summary:
Ref T13210. Minor usability improvements to "bin/bulk export":

- Allow `--class task` to work (previously, only `--class ManiphestTaskSearchEngine` worked).
- If you run `--query jXIlzQyOYHPU`, don't require `--class`, since the query identifies the class on its own.
- Allow users to call `--query A --query B --query C` and get a union of all results.

Test Plan:
- Ran `--class task`, `--query A --query B`…