Enable symmetric merge as the default 'svn merge' behaviour when there is one source, no revision range, no --reintegrate option, and no --ignore-ancestry option. Ignore the '--symmetric' option.
* subversion/include/private/svn_client_private.h (svn_client__symmetric_merge_t): Move structure definition to here ...
* subversion/libsvn_client/merge.c (svn_client__symmetric_merge_t): ... from here. (do_symmetric_merge_locked): Reject unsupported options in reintegrate mode.
* subversion/svn/merge-cmd.c (symmetric_merge): If a reintegrate-like merge is required, reject options that are unsupported options in reintegrate mode. (svn_cl__merge): Do a symmetric merge if there is one source and no revision range, '--reintegrate' or '--ignore-ancestry' options.
* subversion/tests/cmdline/merge_tests.py (dont_merge_revs_into_subtree_that_predate_it): Mark as XFail, because of issue #4217 "Symmetric merge changes merge range notification (merge_tests 78)".
* subversion/tests/cmdline/merge_reintegrate_tests.py (multiple_reintegrates_from_the_same_branch): Specify the revision range explicitly for a record-only merge, since this merge would otherwise be a reintegrate-like symmetric merge, for which record-only is no longer allowed.