chore: slightly optimise rename detection (ref #10777) (#10819)

Only run rename detection for new files. This skips an expensive check
for all updates to existing files. The tradeoff is that we no longer
immediately detect renames on top of another file as a rename -- this
may instead become a copy+delete operation on the destination.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/scanner/walk.go

@@ -471,6 +471,8 @@ func (w *walker) walkRegular(ctx context.Context, relPath string, info fs.FileIn
 	f.RawBlockSize = int32(blockSize)
 	l.Debugln(w, "checking:", f)
 
+	f.New = !hasCurFile
+
 	if hasCurFile {
 		if curFile.IsEquivalentOptional(f, protocol.FileInfoComparison{
 			ModTimeWindow:   w.ModTimeWindow,
@@ -518,6 +520,8 @@ func (w *walker) walkDir(ctx context.Context, relPath string, info fs.FileInfo,
 	f.NoPermissions = w.IgnorePerms
 	l.Debugln(w, "checking:", f)
 
+	f.New = !hasCurFile
+
 	if hasCurFile {
 		if curFile.IsEquivalentOptional(f, protocol.FileInfoComparison{
 			ModTimeWindow:   w.ModTimeWindow,
@@ -570,6 +574,8 @@ func (w *walker) walkSymlink(ctx context.Context, relPath string, info fs.FileIn
 	f = w.updateFileInfo(f, curFile)
 	l.Debugln(w, "checking:", f)
 
+	f.New = !hasCurFile
+
 	if hasCurFile {
 		if curFile.IsEquivalentOptional(f, protocol.FileInfoComparison{
 			ModTimeWindow:   w.ModTimeWindow,