ui: label prompt and echo messages I'm going to add a dedicated command-server channel for status messages, which carries metadata alongside a message text. 'ui.*' label provides a hint how message text should be processed.

ui: add inner function to select write destination I'm going to add a config knob to redirect any status messages to stderr. This function helps to switch underlying file objects. # no-check-commit because of existing write_err() function

ui: remove _write() and _write_err() functions

ui: move pre/post processes from low-level write()s to _writenobuf() This helps adding a dedicated stream for status/error messages. I don't want to add _write*() function per stream.

ui: pass in file object to _writenobuf() See the subsequent patches for why. The "if" block in _writenobuf() will be removed soon.

perf: fix perfrevlogrevisions --reverse Currently, 'endrev' equals `len(revlog)`, a revision that does not exist. When asking for the reverse order, the arguments passed to xrange are `xrange(len(revlog), startrev)` which then crash. We need to offset 'endrev' by one so we don't crash anymore. Also, we offset 'startrev' to ensure we get the same number of revisions with and without the `--reverse` option. Differential Revision: https://phab.mercurial-scm.org/D5228

procutil: import concerns about creationflags on Windows from D1701 I don't have the need anymore for the change in D1701 nor the time to investigate the changes on all supported Windows platforms. I import the stuff I learned on D1701 in the `runbgcommand` so the next people working on it can starts from there. Differential Revision: https://phab.mercurial-scm.org/D5229