gitk: Un-hide selection in areas with non-default background color

The recently introduced background for the tags that highlight added and removed text takes precedence over the background color of the selection. But selected text is more important than the highlighted text. Make the highlighting tags the lowest priority. The same argument holds for the file separator and the highlight of search results. Therefore, make them also low-priority. But search results are a bit more important; therefore, keep them above the other tags. Signed-off-by: Johannes Sixt <j6t@kdbg.org> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>