1) First of all, starting from r1833621, these functions were patched to stop checking symlinks on Windows. The root cause for this change has been an incorrect implementation of stat in APR that didn't properly distinguish between various types of reparse points, some of which are not symlinks.
Providing the custom implementation allows us to remove the hack and properly handle such reparse points irrespectively of the APR version in use.
2) These APIs are used in various performance-critical code paths such as in the core part of `svn st`.
On Win32 the necessary answers for these particular functions can be obtained with a single call to GetFileAttributes(), which is much faster than using the generic stat implementations from APR 1.6.x and 1.7.x (I believe that it would be even slower in the latter case).
A couple of quick tests show about 20%-25% improvement in the speed of `svn st` for a large working copy. The improvement may be more significant with indexers or virus scanners, as just asking for file attributes may completely avoid opening a file (by retrieving the result through the fast I/O QueryOpen).
* subversion/libsvn_subr/io.c (FILE_ATTRIBUTE_TAG_INFO, FileAttributeTagInfo): Add these definitions for old versions of Windows SDK. (typedef GetFileInformationByHandleEx_t, get_file_information_by_handle_ex_proc): New. (win_init_dynamic_imports): Import `GetFileInformationByHandleEx()`. (win32_get_file_information_by_handle): New helper function. (io_win_check_path): New helper with the Win32 implementation required for the "check path" functions. (svn_io_check_path, svn_io_check_resolved_path, svn_io_check_special_path): Invoke the new helper. (io_check_path): Undo the workaround from r1833621 that stopped passing APR_FINFO_LINK when performing a stat.