IsPrefixOf compares segment strings, not directory depth, for paths with leading ".." #226

IsPrefixOf decides containment by comparing normalized path strings segment by segment. That is correct as long as neither path begins with a .. reference — but .. moves up, so a string prefix stops implying containment:

new LocalPath("..").IsPrefixOf(new LocalPath("foo"));    // false, though foo is CWD/foo, which is under ..
new LocalPath("..").IsPrefixOf(new LocalPath("../.."));  // true, though ../.. is above .., not under it

Answering these correctly means modelling how far above the current directory a path starts — the number of leading .. references — and comparing that together with the remaining segments, rather than comparing strings.

This affects AbsolutePath too: /.. survives normalization, so new AbsolutePath("/..") is a constructible value.

Both behaviors predate #223. A closely related case was fixed there: the empty path (the current directory) was reported as a prefix of ../evil.