Fix issue #4537 "mergeinfo parsing inconsistency: empty path". When parsing merginfo, allow the source path to be the empty string, for consistency.
This check was added in r868625. Since r1182771, all other non-canonical paths have been allowed and canonicalized by the parser for issue 4032 "Be graceful on non-canonical paths in mergeinfo".
* subversion/libsvn_subr/mergeinfo.c (parse_pathname): Don't throw an error if the mergeinfo path is empty.
* subversion/tests/libsvn_subr/mergeinfo-test.c (NBR_MERGEINFO_VALS, mergeinfo_vals, mergeinfo_paths, mergeinfo_ranges): Add an empty-path test case here. (NBR_BROKEN_MERGEINFO_VALS, broken_mergeinfo_vals): Remove the empty-path test case here.
Revert r1508225, which moved the include of svn_private_config.h before all includes of our public headers. Public headers shouldn't depend on private headers and all changes that required this are long reverted on trunk.