copies: separate added/removed files by newline instead of null
This makes it more consistent with how we encode copies
(newline-separated lists of null-separated pairs). This perhaps makes
{extras} a little less readable (?) despite avoiding the escaping. I
don't know how I feel about this patch. I'm okay with it being queued
or dropped.
Differential Revision: https://phab.mercurial-scm.org/D6486
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