Refactor some code to allow reusing some code for different scenarios.

Add several comments on how we currently handle wc-repos diffs for
copied paths... (This behavior wasn't introduced by any recent changes.
It is just how 1.8 works).

* subversion/libsvn_client/diff.c
  (diff_repos_wc): Combine code between against BASE and against copy
    scenarios. Handle explicit -R BASE diff to a copy as against BASE.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1570667 13f79535-47bb-0310-9956-ffa450edef68