MFC r264343:
  Add SRC_UPDATE_SKIP, DOC_UPDATE_SKIP, and PORTS_UPDATE_SKIP
  variables.  These are intended to allow bypassing the
  'svn co /usr/{src,doc,ports}' step in the chroot when the
  tree exists from external means.

  The use case here is that /usr/src, /usr/doc, and /usr/ports
  in the chroot exist as result of zfs dataset clones, so it
  is possible (and happens quite often) that the included
  distributions may not be consistent.  (This is not the case
  for -RELEASE builds, but does happen for snapshot builds.)

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