Parse ignore files without the scanner line limit

[andrew]

Ignore files with a line longer than the scanner limit silently lost every later rule, even when that line was a comment. Iterate over the loaded bytes so all lines are parsed, preserving CRLF handling and source line numbers.

Adds regression coverage against Git for long patterns and comments through AddPatterns, AddFromFile, and New.

Fixes #28.

[Copilot]

🔵 Needs a closer look

Preserve lone-CR-terminated patterns by stripping \r only when it follows a newline.

Pull request overview

This PR updates ignore-file parsing to support lines exceeding the scanner limit while preserving line metadata and newline handling.

Changes:

• Replaces scanner-based parsing with byte-based iteration.
• Adds regression tests and benchmarks for long lines, comments, CRLF, and loading paths.

File summaries

File	Summary
long_lines_test.go	Adds long-line regression coverage and benchmarks.
gitignore.go	Implements unrestricted byte-based parsing. Moderate issue (1 vote): trailing \r is removed from final lines without \n, changing existing behavior.

Review details

Suppressed comments (1)

gitignore.go:438

• This unconditionally removes a trailing \r, including when the input's final line has no \n. That changes the existing bufio.ScanLines behavior for a lone-CR-terminated pattern (foo\r becomes foo), which can change both matching and reported pattern text. Only strip \r when bytes.Cut found a newline, i.e. for CRLF.

		raw = bytes.TrimSuffix(raw, []byte{'\r'})

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

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