Let get_tag_by_name return inputted Tag right away.

For caller's convenience most functions assume you are passing
name as a string, but if you already have the Tag we'll just use it.

etiquette/photodb.py

@@ -819,7 +819,9 @@ class PDBTagMixin:
 
     def get_tag_by_name(self, tagname):
         if isinstance(tagname, objects.Tag):
-            tagname = tagname.name
+            if tagname.photodb == self:
+                return tagname
+            tagname = tagname.tagname
 
         tagname = tagname.strip('.+')
         tagname = tagname.split('.')[-1].split('+')[0]