Fix issue #3843 ("file externals cause non-inheritable mergeinfo") on trunk. (A fix was already made for the 1.6.x line.)
* subversion/include/private/svn_wc_private.h (svn_wc__path_switched): Remove as unused. (svn_wc__get_mergeinfo_walk_info): Add 'is_switched' and 'is_file_external' return parameters.
* subversion/libsvn_wc/lock.c (svn_wc__path_switched): Remove as unused.
* subversion/libsvn_wc/node.c (svn_wc__get_mergeinfo_walk_info): Add 'is_switched' and 'is_file_external' return parameters (and do the needful to return them accurately).
* subversion/libsvn_client/merge.c (get_mergeinfo_walk_cb): Update call to svn_wc__get_mergeinfo_walk_info(), losing logic that's now encapsulated in that function and making use of its is-file-external determination.
* subversion/tests/cmdline/externals_tests.py (merge_target_with_externals): Drop @XFail() decorator.