copies: pass changectx instead of manifest to _computenonoverlap
The _computenonoverlap function takes two manifests to allow extensions to hook
in and read the manifest nodes produced by the function. The remotefilelog
extension actually needs the entire changectx instead (which includes the
manifest) so it can prefetch the subset of files necessary for a sparse checkout
(and the sparse checkout depends on which commit is being accessed, hence the
need for the changectx).
I have tests in the remotefilelog extension that cover this.
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.