lib/ignore: Optimise ignoring directories for filesystem watcher (fixes #9339) (#9340)

This improves the ignore handling so that directories can be fully
ignored (skipped in the watcher) in more cases. Specifically, where the
previous rule was that any complex `!`-pattern would disable skipping
directories, the new rule is that only matches on patterns *after* such
a `!`-pattern disable skipping. That is, the following now does the
intuitive thing:

```
/foo
/bar
!whatever
*
```

- `/foo/**` and `/bar/**` are completely skipped, since there is no
chance anything underneath them could ever be not-ignored
- `!whatever` toggles the "can't skip directories any more" flag
- Anything that matches `*` can't skip directories, because it's
possible we can have `whatever` match something deeper.

To enable this, some refactoring was necessary:

- The "can skip dirs" flag is now a property of the match result, not of
the pattern set as a whole.
- That meant returning a boolean is not good enough, we need to actually
return the entire `Result` (or, like, two booleans but that seemed
uglier and more annoying to use)
- `ShouldIgnore(string) boolean` went away with
`Match(string).IsIgnored()` being the obvious replacement (API
simplification!)
- The watcher then needed to import the `ignore` package (for the
`Result` type), but `fs` imports the watcher and `ignore` imports `fs`.
That's a cycle, so I broke out `Result` into a package of its own so
that it can be safely imported everywhere in things like `type Matcher
interface { Match(string) result.Result }`. There's a fair amount of
stuttering in `result.Result` and maybe we should go with something like
`ignoreresult.R` or so, leaving this open for discussion.

Tests refactored to suit, I think this change is in fact quite well
covered by the existing ones...

Also some noise because a few of the changed files were quite old and
got the `gofumpt` treatment by my editor. Sorry not sorry.

---------

Co-authored-by: Simon Frei <freisim93@gmail.com>

lib/fs/basicfs_watch.go

@@ -37,18 +37,14 @@ func (f *BasicFilesystem) Watch(name string, ignore Matcher, ctx context.Context
 		eventMask |= permEventMask
 	}
 
-	if ignore.SkipIgnoredDirs() {
-		absShouldIgnore := func(absPath string) bool {
-			rel, err := f.unrootedChecked(absPath, roots)
-			if err != nil {
-				return true
-			}
-			return ignore.Match(rel).IsIgnored()
+	absShouldIgnore := func(absPath string) bool {
+		rel, err := f.unrootedChecked(absPath, roots)
+		if err != nil {
+			return true
 		}
-		err = notify.WatchWithFilter(watchPath, backendChan, absShouldIgnore, eventMask)
-	} else {
-		err = notify.Watch(watchPath, backendChan, eventMask)
+		return ignore.Match(rel).CanSkipDir()
 	}
+	err = notify.WatchWithFilter(watchPath, backendChan, absShouldIgnore, eventMask)
 	if err != nil {
 		notify.Stop(backendChan)
 		if reachedMaxUserWatches(err) {