sidedatacopies: only store an entry if it has values This will make for a shorter storage and help use to write faster code in simple case. This change already provided a speed boost for copy tracing. For example here is combined time of running copies tracing on a 6989 pairs of revision on the pypy repos: before: 771s after: 631s - 18% This also has a very positive impact on changelog size. For example here are the number for the `00changelog.d` file of pypy. before: 30449712 Bytes after: 24973718 Bytes - 18% This give an overall quite acceptable overhead for storing copies into the changelog: filelog-only: 23370586 sidedata: 24973718 Bytes + 7% Differential Revision: https://phab.mercurial-scm.org/D7068

sidedatacopies: only read from copies when in this mode If we know we expect data from sidedata, we read them from sidedata and nothing else. This avoid looking into extra for revision without sidedata entries. Such revision will be introduced in the next changesets. Differential Revision: https://phab.mercurial-scm.org/D7067

dirs: remove mutable string optimization at all As far as I can see, the optimization trick has been dead since 42e89b87ca79 "dirs: speed up by storing number of direct children per dir". After 42e89b87ca79, the key variable is cleared to NULL at each iteration.

py3: prevent comparison with None in curses histedit (issue6196)