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/model/model.go

@@ -1551,7 +1551,7 @@ func (m *Model) internalScanFolderSubdirs(folder string, subs []string) error {
 					batch = batch[:0]
 				}
 
-				if ignores.Match(f.Name).IsIgnored() || symlinkInvalid(folder, f) {
+				if !f.IsInvalid() && (ignores.Match(f.Name).IsIgnored() || symlinkInvalid(folder, f)) {
 					// File has been ignored or an unsupported symlink. Set invalid bit.
 					l.Debugln("setting invalid bit on ignored", f)
 					nf := protocol.FileInfo{