copies: refactor checkcopies() into a top level method...
copies: refactor checkcopies() into a top level method
This moves checkcopies() out of mergecopies() and makes it a top level
function in the copies module. This allows extensions to override it. For
example, I'm developing a filelog replacement that doesn't have rev numbers
so all the rev number dependent implementation here needs to be replaced
by the extension.
No logic is changed in this commit.