perf: copyedit a few documentation strings Differential Revision: https://phab.mercurial-scm.org/D6170

shelve: add --keep to list of allowables

perf: introduce a `perf.run-limits` options This options make it possible to configure the number of run that the extensions will perform. This is useful for automated benchmark or for performance measurement that need better accuracy.

perf: pass limits as a function argument The function applying the limit has no access to the configuration. Therefore, some higher layer will have to pass it as argument. We do this in an independent change to clarify the next change.

perf: more flexible implementation for checking stop conditions We want to make this logic simpler to configure. The first step is to stop hard-coding every values.

perf: document perfparents

perf: document config options We have configuration, so we better document it.

tests: use "perf" as a the extension name in test-contrib-perf.t This is simpler and seems more "inline" with the name people usually use for this extensions.

shelve: do not update when keeping changes, just move the dirstate This is to leave the working directory unchanged. We reuse the match object as an optimisation.

shelve: refactor _shelvecreatedcommit's match object into calling site We might need to use this match object again to move the dirstate in case the user requested to `--keep` the changes.