phabricator: add a small language to query Differential Revisions 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 with that. 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`. Test Plan: Try querying this Phabricator instance: hg phabread 1+2 # 1, 2 hg phabread D2+D1 # 2, 1 hg phabread ':118-115+:2-1' # 114, 116, 117, 118, 2 hg phabread '((:118-(D115+117)))&:117' # 114, 116 hg phabread ':2&:117' --debug # differential.query is called only once Make sure the output is expected and prefetch works. Differential Revision: https://phab.mercurial-scm.org/D125