Add .sync-exclude.lst to exclude files

Add .sync-exclude.lst to exclude files

Previously the .sync-exclude.lst file of the sync root directory was
not added to the exclude files, because the current logic did only
recognize .sync-exclude.lst files when their containing directory was
discovered during the discovery phase. Therefore the sync root
.sync-exclude.lst file was never discovered. See also
ExcludedFiles::traversalPatternMatch().

Fix: #3830, #2728

Signed-off-by: Felix Weilbach <felix.weilbach@nextcloud.com>