Fix trailing-space trimming after escaped backslashes

[Str0k]

GitIgnoreBasicPattern and GitIgnoreSpecPattern preserve a trailing space whenever the preceding character is a backslash. With an even number of backslashes, Git treats that space as unescaped and trims it. This patch checks backslash parity before deciding whether to preserve the space.

A pattern containing two backslashes followed by a space should match a filename ending in one literal backslash. The new regression module covers both pattern classes and PathSpec; odd backslash runs continue to preserve escaped spaces. The behavior was also compared locally against git check-ignore and across the simple, re2 and hyperscan backends.

Validation: a new regression test fails on the unchanged base and passes with this patch; reproduced in an independent clean checkout.

Full-suite results (including pre-existing failures, if any):

{
  "base": {
    "exit": 1,
    "tests": 881,
    "failures": 6,
    "errors": 0,
    "skipped": 0,
    "failed_cases": [
      "tests.test_07_gitignore_trailing_space.TrailingSpaceAfterEscapedBackslashTest.test_00_even_backslash_run_trailing_space_stripped",
      "tests.test_07_gitignore_trailing_space.TrailingSpaceAfterEscapedBackslashTest.test_01_four_backslashes_trailing_space_stripped",
      "tests.test_07_gitignore_trailing_space.TrailingSpaceAfterEscapedBackslashTest.test_04_regex_normalization",
      "tests.test_07_gitignore_trailing_space.TrailingSpaceAfterEscapedBackslashTest.test_05_pathspec_end_to_end"
    ]
  },
  "patch": {
    "exit": 0,
    "tests": 881,
    "failures": 0,
    "errors": 0,
    "skipped": 0,
    "failed_cases": []
  }
}

AI assistance: implementation and review used Hermes with self-hosted GLM 5.3. Automated test evidence was checked separately. This does not represent a human review.

Additional checks were reproduced on fresh base and patched checkouts:

Check	Base	Patch
mypy	2 type errors	2 type errors
pyright	2 type errors	2 type errors
unittest	221 tests; exit 1	221 tests; exit 0

The type-checker errors below are also present on the unchanged base; this patch adds none.

Pre-existing type-checker diagnostics

{
  "mypy": [
    {
      "file": "pathspec/_typing.py",
      "code": "no-redef",
      "message": "Name \"deprecated\" already defined (possibly by an import)"
    },
    {
      "file": "pathspec/_typing.py",
      "code": "return-value",
      "message": "Incompatible return value type (got \"_Wrapped[[VarArg(Any), KwArg(Any)], Any, [VarArg(Any), KwArg(Any)], Any]\", expected \"F\")"
    }
  ],
  "pyright": [
    {
      "file": "pathspec/_typing.py",
      "code": "reportAssignmentType",
      "message": "Type \"type[deprecated]\" is not assignable to declared type \"(message: str, /, *, category: type[Warning] | None = DeprecationWarning, stacklevel: int = 1) -> ((F@deprecated) -> F@deprecated)\"\n\u00a0\u00a0Type \"type[deprecated]\" is not assignable to type \"(message: str, /, *, category: type[Warning] | None = DeprecationWarning, stacklevel: int = 1) -> ((F@deprecated) -> F@deprecated)\"\n\u00a0\u00a0\u00a0\u00a0Parameter 1: type \"str\" is incompatible with type \"LiteralString\"\n\u00a0\u00a0\u00a0\u00a0\u00a0\u00a0\"str\" is not assignable to \"LiteralString\""
    },
    {
      "file": "pathspec/_typing.py",
      "code": "reportReturnType",
      "message": "Type \"_Wrapped[..., Any, ..., Any]\" is not assignable to return type \"F@deprecated\"\n\u00a0\u00a0Type \"_Wrapped[..., Any, ..., Any]\" is not assignable to type \"F@deprecated\""
    }
  ]
}

[cpburnz]

This bug is very obvious now that you point it out. Thanks for fixing it.