phabricator: refactor `phabread` to write all patches at once This will be necessary to create a first class `phabimport` command. That command requires a transaction, and will import all named patches within a single transaction. But if Phabricator queries also happen within the transaction, that leaves open the chance that an exception is raised, the transaction is abandoned, and the next command that is run will complain about needing to run `hg recover`. Differential Revision: https://phab.mercurial-scm.org/D8135