phabricator: allow multiple DREVSPEC args to phabread|phabimport|phabupdate Per Augie's request. Like regular revsets, the result is the union of all given specs. Unlike regular revsets, these don't resolve in parent -> child order, and should be specified as such on the command line. This change invalidated a previous test using an empty `hg phabread`, so it has been switched to `hg debugcallconduit` to preserve that coverage. Differential Revision: https://phab.mercurial-scm.org/D8233