Handle backslash escapes in gitignore bracket expressions

[gyanu2507]

A backslash inside a gitignore bracket expression escapes the next character in git, the same as it does outside one. pathspec treats it as a literal backslash instead, so several valid patterns match the wrong files:

pattern	git matches	pathspec matched
[a\-c]	a, -, c	a, b, c, \, ], ^ (read as the range \-c)
[\]]	]	\]
[\]a]	], a	nothing
[!\]]	any one character except ]	]]
[\n]	n	n, \

The git column comes from git ls-files -o -i --exclude-standard in a scratch repo with core.ignorecase=false (git 2.50.1). ignore on npm agrees with git on all of these.

Cause

_translate_segment_glob() had two problems with escapes inside [...]:

• The scan for the closing ] didn't skip escaped characters, so [\]] ended at the escaped ].
• The bracket body was built with pattern[i:j].replace('\\', '\\\\'), which turned every backslash into a literal backslash member of the set.

Fix

• The closing bracket scan now steps over \ and the character after it.
• The bracket body is translated with one regex that matches either a POSIX class token or an escaped character, left to right. An escaped character becomes re.escape(char). The existing _translate_posix_class() still handles classes. Because escapes are consumed in the same pass, [\[:alpha:]] is the literal set [, :, a, l, p, h followed by ], which is what git does too.

This lives in the shared base translator, so it applies to both GitIgnoreSpecPattern and GitIgnoreBasicPattern.

Tests

Added test_13_bracket_escape to tests/test_04_gitignore_spec.py and test_14_bracket_escape to tests/test_03_gitignore_basic.py. Every expected result was checked against git first.

python -m unittest \
  tests.test_04_gitignore_spec.GitIgnoreSpecPatternTest.test_13_bracket_escape \
  tests.test_03_gitignore_basic.GitIgnoreBasicPatternTest.test_14_bracket_escape

Without the fix, 32 subtests fail. With it, both tests pass.

The full suite passes with python -m unittest on Python 3.13 with hyperscan and google-re2 installed (243 tests, 1 skipped). I also ran [a\-c], [\]], [!\]], [\[:alpha:]], [\#x] and [\ a] through GitIgnoreSpec and PathSpec('gitignore') on the simple, re2 and hyperscan backends, and all three give git's answers. mypy and pyright output is unchanged from master.

Out of scope: reversed ranges such as [z-a] or [a-\]] are still discarded the same way as on master. git matches the start character in that case, which is a separate difference.

[cpburnz]

Thanks for finding and fixing this!