copies: follow copies across merge base without source file (issue6163) As in the previous patch, consider these two histories: @ 4 'rename x to y' | o 3 'add x again' | o 2 'remove x' | | o 1 'modify x' |/ o 0 'add x' @ 4 'rename x to y' | o 3 'add x again' | | o 2 'modify x' | | | o 1 'add x' |/ o 0 'base' We trace copies from the 'modify x' commit to commit 4 by going via the merge base (commit 0). When tracing file 'y' (_tracefile()) in the first case, we immediately find the rename from 'x'. We check to see if 'x' exists in the merge base, which it does, so we consider it a valid copy. In the second case, 'x' does not exist in the merge base, so it's not considered a valid copy. As a workaround, this patch makes it so we also attempt the check in mergecopies's base commit (commit 1 in the second case). That feels pretty ugly to me, but I don't have any better ideas. Note that we actually also check not only that the filename matches, but also that the file's nodeid matches. I don't know why we do that, but it was like that already before I rewrote mergecopies(). That means that the rebase will still fail in cases like this (again, it already failed before my rewrite): @ 4 'rename x to y' | o 3 'add x again with content X2' | o 2 'remove x' | | o 1 'modify x to content X2' |/ o 1 'modify x to content X1' | o 0 'add x with content X0' Differential Revision: https://phab.mercurial-scm.org/D6604