--delete-empty-revs breaks copy operations (and --renumber-revs)

[emosenkis]

Copy operations may point to revisions that were deleted (even though the paths they refer to are included) because they may point to a revision in which the target path was not modified (any revision between the desired modification and the next modification).

This breaks --renumber-revs because it tries to look up the copy target rev in revmap but the key is missing.

To fix this, we'll have to lookup the greatest key in revmap that does not exceed X instead of just looking for X.