copies: clarify mutually exclusive cases in _chain() with a s/if/elif/ If the 'b' dict has a rename from 'x' to 'y', it shouldn't be possible for 'x' to be both (a key) in 'a' and in 'src'. That would mean that 'x' is a file in the source commit and also a rename destination in the intermediate commit. But we currently don't allow renaming files onto existing files, so that shouldn't happen. So let's clarify that by using an "elif" instead of an "if". And if we did allow renaming files onto existing files, we should prefer to use the rename destination in the intermediate commit as source anyway. Differential Revision: https://phab.mercurial-scm.org/D6276