Honor gitignore negation patterns for directories

[cryo2010]

Fixes #1563

Problem

The common .gitignore idiom for ignoring extensionless files (POSIX binaries):

*
!*.*
!*/

caused ag to skip essentially the entire repo. Git interprets it as "ignore everything, then un-ignore anything with a dot and all directories," but ag matched directories against ignore patterns by plain name first, so * ignored them before the trailing-slash form that !*/ can match was ever consulted. Negation matches also could not take precedence: path_ignore_search returned the same 0 for "invert pattern matched" and "nothing matched."

Fix

• path_ignore_search now returns a tri-state: 1 (ignored), -1 (explicitly un-ignored by an invert pattern), 0 (no match). Invert patterns are checked before the other pattern classes at the same level, and the ackmate filter result is normalized to 0/1 so it does not collide with the new -1.
• filename_filter evaluates a directory's plain and trailing-slash forms together, so an invert match on either form wins. An explicit un-ignore stops the walk up the parent ignore chain, matching git's deepest-match precedence.

Testing

• New cram test tests/ignore_invert_dir.t covering the idiom above; it fails on master and passes with this change.
• Full test suite passes (44 tests, 1 skipped).
• Manually verified against git status --porcelain --ignored on a test repo with nested directories and extensionless files at multiple depths: ag's results now match git's ignore semantics exactly.