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copies: clarify mutually exclusive cases in _chain() with a s/if/elif/

Authored by martinvonz.

Description

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