Don't create two unneeded ra sessions on every 'svn import' without
--no-ignore. Also avoid a race condition on the HEAD revision by passing
the explicit revision to several ra operations, to make sure they all look
at the same revision.

* subversion/libsvn_client/import.c
  (import): Add base_rev argument and pass it for the root open operation.
  (svn_client_import5): Obtain the base revision, properties and inherited
    properties on the primary ra session instead of via two separate sessions.
    Add note that we still open an ra session for the autoprops.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1482962 13f79535-47bb-0310-9956-ffa450edef68