lib/model: Don't set ignore bit when it's already set

This adds a metric for "committed items" to the database instance that I
use in the test code, and a couple of tests that ensure that scans that
don't change anything also don't commit anything.

There was a case in the scanner where we set the invalid bit on files
that are ignored, even though they were already ignored and had the
invalid bit set. I had assumed this would result in an extra database
commit, but it was in fact filtered out by the Set... Anyway, I think we
can save some work on not pushing that change to the Set at all.

GitHub-Pull-Request: https://github.com/syncthing/syncthing/pull/3298

lib/db/set_test.go

@@ -624,6 +624,39 @@ func TestLongPath(t *testing.T) {
 	}
 }
 
+func TestCommitted(t *testing.T) {
+	// Verify that the Committed counter increases when we change things and
+	// doesn't increase when we don't.
+
+	ldb := db.OpenMemory()
+
+	s := db.NewFileSet("test", ldb)
+
+	local := []protocol.FileInfo{
+		{Name: string("file"), Version: protocol.Vector{{ID: myID, Value: 1000}}},
+	}
+
+	// Adding a file should increase the counter
+
+	c0 := ldb.Committed()
+
+	s.Replace(protocol.LocalDeviceID, local)
+
+	c1 := ldb.Committed()
+	if c1 <= c0 {
+		t.Errorf("committed data didn't increase; %d <= %d", c1, c0)
+	}
+
+	// Updating with something identical should not do anything
+
+	s.Update(protocol.LocalDeviceID, local)
+
+	c2 := ldb.Committed()
+	if c2 > c1 {
+		t.Errorf("replace with same contents should do nothing but %d > %d", c2, c1)
+	}
+}
+
 func BenchmarkUpdateOneFile(b *testing.B) {
 	local0 := fileList{
 		protocol.FileInfo{Name: "a", Version: protocol.Vector{{ID: myID, Value: 1000}}, Blocks: genBlocks(1)},