copies: filter out copies grafted from another branch Consider this simple history: ``` @ 3 modify y | o 2 copy x to y, modify x | | o 1 copy x to y, modify x |/ o 0 add x ``` If we now rebase commit 3 onto 1, Mercurial will look for copies between commit 2 and commit 1. It does that by going backwards from 2 to 0 and then forwards from 0 to 1. It will find that x was copied to y, since that was what happened on the path between them (namely in commit 1). That leads Mercurial to do a 3-way merge between y@3 and y@1 with x@2 as base. We want to use y@2 as base instead. That's also what happened until commit 1d6d1a15. This patch fixes the regression by adding another filtering step when chaining copies via a diffbase. The new filtering step removes copies that were the same between the two branches (same source and destination, but not necessarily the same contents). Differential Revision: https://phab.mercurial-scm.org/D10120