PathTool.getRelativeFilePath: fix broken Windows drive-letter regex

[elharo]

The regex ^\\[a-zA-Z]: at lines 146 and 149 had two bugs:

1. The \\[ matched a literal [ character, not a backslash, because [ was escaped by \
2. String.matches() requires the entire string to match the pattern, but this regex was intended to match only the leading \C: prefix — so it never matched any real path

The Windows drive-letter normalization code was effectively dead. Paths like \C:\foo (produced by File.getPath() on Windows) were never getting their leading \ stripped, causing the drive-letter comparison logic at lines 154–175 to be skipped. This could produce incorrect relative paths or fail to detect mismatched drives (returning a path instead of null).

Fix: Replaced the broken regex with a simple character-level check:

if (toPath.length() > 2 && toPath.charAt(0) == '\\'
        && Character.isLetter(toPath.charAt(1)) && toPath.charAt(2) == ':')

Fixes #383

[Copilot]

Pull request overview

This PR fixes Windows drive-letter handling in PathTool.getRelativeFilePath() by replacing a broken String.matches()-based regex with a direct character-level prefix check, restoring the intended drive normalization and cross-drive mismatch behavior.

Changes:

• Replaced the non-matching Windows drive-letter regex with a simple charAt/Character.isLetter prefix check for \C:-style paths.
• Added a regression test ensuring paths on different Windows drives (with a leading backslash) return null.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/main/java/org/apache/maven/shared/utils/PathTool.java	Replaces the broken regex-based Windows drive-letter detection with a character-level check to correctly strip a leading \ before C:.
src/test/java/org/apache/maven/shared/utils/PathToolTest.java	Adds a regression test covering cross-drive relative path behavior when paths include a leading backslash before the drive letter.

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