In symmetric merge, defer the target WC checks for mixed-rev, local mods and switched subtrees until after determining whether a reintegrate-like merge is required, so that we can force doing the strict checks if so.
* subversion/include/private/svn_client_private.h (svn_client__symmetric_merge_t): Add flags to store the requested allow_mixed_rev, allow_local_mods, allow_switched_subtrees options.
* subversion/libsvn_client/merge.c (svn_client__find_symmetric_merge): Don't check here; store the options instead. (do_symmetric_merge_locked): Do the requested checks here, or if it's a reintegrate-like merge then force all three of the checks.
* subversion/svn/merge-cmd.c (svn_cl__merge): Simplify the code slightly, as we know that the 'reintegrate' option is false in this block.