Make SVN fingerprint opens race-safe

[lzl51230]

Related to #1113.

The SVN fingerprint path checks run before ordinary fopen/opendir calls. A concurrent path swap can therefore replace a previously validated regular file or directory with a symlink, reparse point, special file, or path outside the working-copy root before it is opened.

Use no-follow, handle-relative opens where supported, or validate the opened handle identity and containment before reading. Add a regression test that swaps the candidate between validation and open.

[DeusData]

Tracked as a 0.9.2-rc path-safety blocker for #1113/#1127. A pre-open path check is not sufficient when the subsequent open can race with a symlink, reparse point, special-file, or containment swap. The regression should exercise the swap at a deterministic test seam and validate the opened handle, not repeat the same pathname check.