worker: avoid reading 1 byte at a time from the OS pipe Apparently `pickle.load` does a lot of small reads, many of them literally 1-byte, so it benefits greatly from buffering. This change enables the buffering, at the cost of more complicated interaction with the `selector` API. On one repository with ~400k files this reduces the time by about ~30s, from ~60 to ~30s. The difference is so large because the actual updating work is parallellized, while these small reads are bottlenecking the central hg process.

diffutil: rewrite an ersatz ternary operator for building diffopts.text Pytype tends to get confused by these, but the real problem here is that if the caller passes `opts={}` as TortoiseHg has been doing, that is set as the value for `diffopts.text` when in reality it expects a boolean. When `None` is passed explicitly, the default value in `mdiff.defaultopts` is assigned.

mdiff: add a __str__ method to diffopts This makes it easier to debug by just formatting the object into `%s` to see the members and state, instead of the class and memory address.