phabricator: change "readpatch" to be more flexible
Previously, `readpatch` and `querydrev` take a same `params` and `stack`
parameters. This patch changes `readpatch` so it takes the output of
`querydrev`, not the input of `querydrev`. This makes the code more
flexible and cleaner.
Differential Revision: https://phab.mercurial-scm.org/D124