copies: move comment about implementation of mergecopies() to end
When you start reading about mergecopies(), you want to know what it
is, not that there are different implementations depending on config,
so this patch moves that comment to the end.
By the way, we don't seem to define what "copytracing" is. I'm just
leaving it that way because I don't know what it is myself. It seems
to be referred to only on mergecopies() (and not in pathcopies(), for
example), so maybe "copytracing" is supposed to be exactly what
mergecopies() does?
Differential Revision: https://phab.mercurial-scm.org/D6235
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