Fix KeyError in detailed_match_files for unmatched negation patterns

[bysiber]

detailed_match_files() raises KeyError when a negation pattern (include=False) matches a file that wasn't previously added by an inclusion pattern.

For example:

from pathspec import PathSpec

# Pattern list where negation is broader than inclusion
spec = PathSpec.from_lines('gitwildmatch', [
    '*.txt',
    '!*.log',
])

# detailed_match_files processes patterns in order:
# 1. '*.txt' (include=True) → adds .txt files to result
# 2. '!*.log' (include=False) → tries to remove .log files → KeyError
spec.match_files(['test.txt', 'test.log'], details=True)

The !*.log negation pattern matches test.log, but that file was never added to the result set by a prior inclusion pattern. The del return_files[file] on line 160 of util.py raises KeyError.

The fix adds a membership check before deletion — if a file isn't in the result set, the negation is simply a no-op, which is consistent with how gitignore processes negation (it can only un-ignore files that were previously ignored).