fix: negated patterns with slashes incorrectly use basename-only matching when matchBase is true

[deepakganesh78]

Fixes #136

Problem

When matchBase: true is set, picomatch.test() unconditionally tests the compiled regex against the basename of the input path. For negated patterns containing path separators (e.g. !**/examples/**), this produces incorrect results because the negative lookahead regex cannot detect directory structure from the basename alone.

Reproduction:

const pm = require('picomatch');
const opts = { matchBase: true, dot: true };

pm.isMatch('packages/pkg-2/examples/.eslintrc.yaml', '!**/examples/**', opts);
// Actual: true (WRONG - path IS inside examples/)
// Expected: false

Root cause

In picomatch.test(), when matchBase is true, the code calls picomatch.matchBase() which tests the regex against utils.basename(input) only. For the negated regex ^(?!^(?:...examples...)$).*$ tested against basename .eslintrc.yaml, the positive pattern doesn't match the basename, so the negative lookahead passes and the result is incorrectly true.

Fix

For negated patterns whose glob (after stripping the ! prefix) contains a path separator (/ or \), the fix now tests both the full path (regex.exec(output)) and the basename (matchBase). If either method detects that the positive pattern matches, the overall result is non-matching (null). This ensures:

• Directory-structure negation patterns like !**/examples/** correctly reject paths inside examples/
• Basename-oriented negation patterns like !**/*.js continue to work (the basename test catches cases where the full-path regex misses due to ./ prefix handling)

Test results

• Baseline (clean checkout): 1977 passing, 0 failing
• After fix: 1978 passing, 0 failing (+1 new regression test)
• No regressions introduced

Compatibility

This change only affects negated patterns that contain path separators when matchBase is enabled. Non-negated patterns and slash-free patterns are completely unaffected.