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