fsmonitor: fix empty path in dirfilter

## Summary

Running the ignore matcher on an empty path produces incorrect results.
In the rust implementation, this isn't an issue, because there are
directory matchers that run on the directory result. In the Python
treestate bindings, we don't have that functionality, so it returns an
empty string.

This makes this logic match the code in `dirstate.py` (line ~925) where
we check for path being an empty string.

This fixes `sl status` (and related functionality) when fsmonitor is
enabled and there's a complex gitignore file that ignores files in the
root directory.

## Test Plan

Added Mercurial integration test

eden/scm/edenscm/ext/fsmonitor/__init__.py

@@ -446,6 +446,8 @@ def _innerwalk(self, match, event, span):
         ignorevisitdir = self.dirstate._ignore.visitdir
 
         def dirfilter(path):
+            if path == "":
+                return False
             result = ignorevisitdir(path.rstrip("/"))
             return result == "all"
 

eden/scm/tests/test-gitignore.t

@@ -70,3 +70,19 @@
   ? exp/.gitignore
   ? exp/i.tmp
   ? x.pyc
+
+# Test exclusion patterns
+
+  $ cat > .gitignore << 'EOF'
+  > /*
+  > !/build
+  > EOF
+
+  $ rm -rf build/
+  $ mkdir build
+  $ touch build/libfoo.so t.tmp Makefile
+
+  $ hg status
+  ? build/libfoo.so
+  $ hg status
+  ? build/libfoo.so

eden/scm/tests/test-status-root-ignored-py.t

@@ -0,0 +1,15 @@
+#require fsmonitor
+
+  $ configure modernclient
+  $ setconfig status.use-rust=False workingcopy.ruststatus=False
+  $ newrepo
+
+Ensure that, when files in the root are ignored and there is an exclusion, that hg status returns the correct value
+  $ echo -e "/*\n!/foobar" > .gitignore
+  $ hg status
+  $ mkdir foobar
+  $ touch root-file foobar/foo # adds files to root and to foobar
+  $ hg status
+  ? foobar/foo
+  $ hg status # run it a second time to ensure that we didn't accidentally exclude the file
+  ? foobar/foo