perf: add `parent-1` as possible source for perfrevlogwrite This source will use a diff against p1 in all case.

perf: add the notion of "source" to perfrevlogwrite We want to test performance associated witch various way to add a new revision. They will be specified using this new argument.

perf: only display the total time for perfrevlogwrite if quiet This provide a simple way to get an overview of the total performance.

perf: offer full details in perfrevlogwrite This will be useful for people who want to study the timing pattern more closely.

perf: introduce a perfrevlogwrite command The command record times taken by adding many revisions to a revlog. Timing each addition, individually. The "added revision" are recreations of the original ones. To time each addition individually, we have to handle the timing and the reporting ourselves. This command is introduced to track the impact of sparse-revlog format on delta computations at initial storage time. It starts with the full text, a situation similar to the "commit". Additions from an existing delta are better timed with bundles. The complaints from `check-perf-code.py` are not relevant. We are accessing and "revlog" opener, not a repository opener.

tests: fix config knob in test-narrow-clone-stream.t Two patches landed in parallel and had a semantic conflict. This resolves the mess and leaves us with passing tests. Differential Revision: https://phab.mercurial-scm.org/D5231

remotefilelog: fix various whitespace issues in docstring Differential Revision: https://phab.mercurial-scm.org/D5230

ui: add config knob to redirect status messages to stderr (API) This option can be used to isolate structured output from status messages. For now, "stdio" (stdout/err pair) and "stderr" are supported. In future patches, I'll add the "channel" option which will send status messages to a separate command-server channel with some metadata attached, maybe in CBOR encoding. This is a part of the generic templating plan: https://www.mercurial-scm.org/wiki/GenericTemplatingPlan#Sanity_check_output .. api:: Status messages may be sent to a dedicated stream depending on configuration. Don't use ``ui.status()``, etc. as a shorthand for conditional writes. Use ``ui.write()`` for data output.

ui: hide fin/fout/ferr attributes behind @property functions This allows keeping references to fout/ferr/fin which are updated when these properties are changed. See the next patch.

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.