Avoid a theoretical case where we call editor->abort_edit() twice.
(Only triggerable by an import, which imported nothing... where the
 first abort fails)

Found by: ivan

* subversion/libsvn_client/import.c
  (import_ctx_t): Remove TODO comment that describes implementation that
    doesn't follow current best practices.
  (import): Add output argument describing whether we committed something.
    Allocate import_ctx on the stack. Remove abort on no-change.
  (svn_client_import5): Use new information to determine when to abort.
    Always destroy iterpool.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1607299 13f79535-47bb-0310-9956-ffa450edef68