perf: add a `pre-run` option sometimes, the initial run is necessary to warm some cache that are not relevant for the current measurement. We add a new `perf.pre-run` option to specify a number of run of the benchmark logic that will happens before measurement are taken.

narrow: consider empty commits to be "inside the narrow spec" for templates It doesn't seem useful to exclude them, or harmful to include them. Users writing log templates using outsidenarrow as a predicate might consider it unexpected if their locally created empty drafts are treated as if they contained something outside the clone. Differential Revision: https://phab.mercurial-scm.org/D6414

rust-python3: useless python2 specific import This python27_sys import prevents building with python3, it had been previously removed in a5fa9140ce4c, but that has been since pruned Differential Revision: https://phab.mercurial-scm.org/D6415