Slashes inside gitignore bracket expressions split the pattern incorrectly #177

Reproduction

On current master (47ea6a5b02bc):

from pathspec import GitIgnoreSpec

spec = GitIgnoreSpec.from_lines(['[a/b].txt'], backend='simple')
print(spec.match_file('a.txt'))  # False
print(spec.match_file('b.txt'))  # False

Git ignores both files with that same rule. For example, in a fresh repository:

printf '[a/b].txt\n' > .gitignore
touch a.txt b.txt
git check-ignore --no-index a.txt b.txt
# a.txt
# b.txt

The slash is inside a bracket expression, so it must not split the expression into two path segments. Git still treats the raw slash as making this rule relative to the ignore file's directory: dir/a.txt must not match this rule. A bracket must never consume an actual path separator either.

Both gitignore pattern factories currently split the original pattern at every /, before translating brackets. This also affects negated brackets and directory-only rules containing such a bracket.

This is distinct from #163, which prevented brackets from matching path separators, and #170, which handled escaped separators between segments. I have a fix with tests for bracket-aware splitting, anchoring, separator exclusion and optional-backend compatibility. No production incident is claimed.