Fix not transferring files that don't differ in size - fixes #911

Due to a logic error files stored on remotes which support modtime but
not hashes weren't being transferred when updating with a file of the
same size but different modtime.  Instead the modtime of the remote
file was being set to that of the local file.

In practice this affected crypt with all remotes except Amazon Drive
and Dropbox.

fs/sync_test.go

@@ -336,6 +336,30 @@ func TestSyncAfterChangingModtimeOnlyWithNoUpdateModTime(t *testing.T) {
 	fstest.CheckItems(t, r.fremote, file2)
 }
 
+func TestSyncDoesntUpdateModtime(t *testing.T) {
+	if fs.Config.ModifyWindow == fs.ModTimeNotSupported {
+		t.Skip("Can't run this test on fs which doesn't support mod time")
+	}
+	r := NewRun(t)
+	defer r.Finalise()
+
+	file1 := r.WriteFile("foo", "foo", t2)
+	file2 := r.WriteObject("foo", "bar", t1)
+
+	fstest.CheckItems(t, r.flocal, file1)
+	fstest.CheckItems(t, r.fremote, file2)
+
+	fs.Stats.ResetCounters()
+	err := fs.Sync(r.fremote, r.flocal)
+	require.NoError(t, err)
+
+	fstest.CheckItems(t, r.flocal, file1)
+	fstest.CheckItems(t, r.fremote, file1)
+
+	// We should have transferred exactly one file, not set the mod time
+	assert.Equal(t, int64(1), fs.Stats.GetTransfers())
+}
+
 func TestSyncAfterAddingAFile(t *testing.T) {
 	r := NewRun(t)
 	defer r.Finalise()