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