Merge the r1575525 group from trunk:

 * r1575525, r1575915
   Resolve issue #4427: File externals can't be exported.
   Justification:
     File externals 'are just files', so why should they be discriminated
     by export?
   Notes:
     r1575525 resolves the most important issues, but in some cases still
     doesn't allow exporting file externals. r1575915 simplifies the fix and
     resolves a few remaining issues.
   Votes:
     +1: rhuijben, breser, philip



git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1591139 13f79535-47bb-0310-9956-ffa450edef68