fix: ignore gitignore patterns with unmatched trailing backslashes

[SergiPantoja]

While using pathspec in a downstream project, I noticed that patterns ending in an unmatched trailing backslash behave differently from Git.

Git documents that a backslash at the end of a gitignore pattern is invalid and never matches:

https://git-scm.com/docs/gitignore#_pattern_format

In testing with Git 2.55.0, Git also continues processing subsequent valid rules after encountering such a pattern.

Currently, both GitIgnoreSpec.from_lines()and PathSpec.from_lines("gitignore", ...) raise GitIgnorePatternError while compiling an unmatched trailing backslash. Because the pattern list is compiled together, this also prevents later valid patterns from being used.

For example, given these .gitignore bytes:

b"fileA\\\n*.log\n"

Git 2.55.0 treats the first rule as non-matching and still applies the second:

'fileA': no match (exit 1)
'fileA\\': no match (exit 1)
'fileA.txt': no match (exit 1)
'fileA.log': .gitignore:2:*.log	fileA.log (exit 0)

Before this change, both Specs instead fail immediately:

GitIgnoreSpec.from_lines():
  GitIgnorePatternError: Invalid git pattern: 'fileA\\'
  ValueError: Escape character found with no next character to escape: 'fileA\\'
  
 PathSpec.from_lines():
  GitIgnorePatternError: Invalid git pattern: 'fileA\\'
  ValueError: Escape character found with no next character to escape: 'fileA\\'

Fix:

• Treat gitignore patterns ending in an unmatched trailing backslash as null patterns in both GitIgnoreBasicPattern and GitIgnoreSpecPattern.
• Preserve valid paired trailing backslashes, where one backslash escapes the other.
• Preserve existing escaped trailing space behavior.

Tests:

• Add direct pattern tests and end-to-end spec tests verifying that later valid patterns remain usable

Ran the test suite locally:

Ran 219 tests in 0.045s

OK (skipped=372)

The behavior was also compared directly against git check-ignore using Git 2.55.0.