fsmonitor: don't apply prefix matching to paths obtained from watchman

If I understand it, watchman returns changed files and directories, and a
directory change doesn't mean we need to scan all files under the directory.

lib/src/local_working_copy.rs

@@ -52,7 +52,7 @@ use crate::fsmonitor::FsmonitorKind;
 use crate::gitignore::GitIgnoreFile;
 use crate::lock::FileLock;
 use crate::matchers::{
-    DifferenceMatcher, EverythingMatcher, IntersectionMatcher, Matcher, PrefixMatcher,
+    DifferenceMatcher, EverythingMatcher, FilesMatcher, IntersectionMatcher, Matcher, PrefixMatcher,
 };
 use crate::merge::{Merge, MergeBuilder, MergedTreeValue};
 use crate::merged_tree::{MergedTree, MergedTreeBuilder};
@@ -863,7 +863,7 @@ impl TreeState {
                         .collect_vec()
                 });
 
-                Some(Box::new(PrefixMatcher::new(&repo_paths)))
+                Some(Box::new(FilesMatcher::new(&repo_paths)))
             }
         };
         Ok(FsmonitorMatcher {