dispatch: move IOError handling and flushing of streams to `dispatch()` Instead of patching both dispatch code and commandserver code, we directly handle this in `dispatch.dispatch()`. Thanks to Yuya who recommended this.

simplemerge: write output only once it's complete `simplemerge()` can write either to `ui.fout` or to the file context (for in-memory merge). This patch simplifies the code a bit by making it build the output the same way regardless of where it's written, and then writes the whole output at once. I don't think it will be a problem that we don't output anything until the whole file is merged even if the file is large. Differential Revision: https://phab.mercurial-scm.org/D9550

simplemerge: avoid quadratic concatenation when building output text I haven't checked if the difference is measurable, but the new version is no less readable or idiomatic, so I don't think performance numbers are needed. Differential Revision: https://phab.mercurial-scm.org/D9549

simplemerge: work with opts as native strings instead of bytes There was little reason to use `pycompat.byteskwargs()` in `simplemerge()` as far as I could tell. Differential Revision: https://phab.mercurial-scm.org/D9548