debugcommands: introduce actions to perform deterministic reads "readavailable" is useful as a debugging device to see what data is available on a pipe. But the mechanism isn't deterministic because what's available on a pipe is highly conditional on timing, system load, OS behavior, etc. This makes it not suitable for tests. We introduce "ereadline," "read," and "eread" for performing deterministic I/O operations (at least on blocking file descriptors). We stop short of converting existing consumers of "readavailable" in tests because we're working out race conditions and deadlocks on Windows. But the goal is to eventually move tests away from "readavailable" to these new APIs. Differential Revision: https://phab.mercurial-scm.org/D2720