Previously, phabread can only be used to read a single patch, or a single
stack of patches. In the future, we want to have more complex queries like
filtering with status (open, accepted, closed, etc), or maybe more complex
like filtering by reviewers etc. The command line flag approach won't scale
Besides, we might want to have other commands to update Differential
Revision status in batch, like accepting a stack using a single command.
Therefore, this patch adds a small language. It has basic set operations:
&, +, - and an ancestor operator to support --stack.