git-remote-entire: case-insensitive core URL comparison via shared helper

Core URL scheme and DNS host are case-insensitive (RFC 3986); the prior comparisons folded only the trailing slash, so a mixed-case aud or issuer would fail to match an otherwise-identical core. Extract auth.EqualCoreURL (trailing-slash- and case-insensitive) and use it for both the ENTIRE_TOKEN trust gate (coreTrusted) and context matching (formerly sameIssuer) so the two agree on core-URL identity. Safe for the trust gate: case folding only makes variants of the same DNS host match, never a different host.