copytrace: add a a new config to limit the number of drafts in heuristics The heuristics options tries to the default full copytracing algorithm if both the source and destination branches contains of non-public changesets only. But this can be slow in cases when we have a lot of drafts. This patch adds a new config option experimental.copytrace.sourcecommitlimit which defaults to 100. This value will be the limit of number of drafts from c1 to base. Incase there are more changesets even though they are draft, the heuristics algorithm will be used. Differential Revision: https://phab.mercurial-scm.org/D763