Add --match-relative-path duplicate filter and document ASCII-only case matching semantics

[Copilot]

• Review new PR feedback and locate affected comparison logic/tests
• Re-apply regression test for --match-relative-path with explicit file inputs of different basenames
• Run targeted tests for updated match-relative-path coverage (blocked: local build requires missing system dependency glib-2.0 >= 2.64)
• Run final code review and CodeQL scan
• Reply to new PR comment with commit hash

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[Copilot]

Pull request overview

This PR introduces a new duplicate-filtering mode, --match-relative-path, to restrict duplicate detection to files that share the same relative path across scanned roots, and updates the manpage to clarify the ASCII-only case-insensitive matching semantics used by these comparisons.

Changes:

• Add --match-relative-path CLI flag and config plumbing.
• Implement relative-path comparison (rm_file_relpaths_cmp) and integrate it into file grouping/comparison.
• Add pytest coverage for relative-path matching and document ASCII-only case-insensitive behavior in the manpage.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_options/test_match_relative_path.py	Adds option-level tests for positive/negative relative-path matching scenarios.
lib/preprocess.c	Applies match_relative_path (and fixes early-return behavior for match_without_extension) during file comparison/grouping.
lib/file.h	Declares the new relative-path comparison helper.
lib/file.c	Implements rm_file_relpaths_cmp by comparing trie nodes up to the file’s traversal depth.
lib/cmdline.c	Registers the new --match-relative-path command-line option.
lib/cfg.h	Adds match_relative_path to runtime configuration.
docs/rmlint.1.rst	Documents the new flag and clarifies ASCII-only case-insensitive matching semantics.

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