Clarify _is_already_pinned: two explicit cases, no None==None revision match

Tag path: return tag equality directly.
Revision path: return False if either revision is absent so that two
unset revisions are never treated as pinned; otherwise require equality
and revision_is_enough().

https://claude.ai/code/session_01Doq8oQtBRH4afusvp9Dv4p

Author: claude

dfetch/project/subproject.py

@@ -463,10 +463,11 @@ def _is_already_pinned(
         self, project: ProjectEntry, on_disk_version: Version
     ) -> bool:
         """Return True if *project* is already pinned to *on_disk_version*."""
-        if project.version.tag and project.version.tag == on_disk_version.tag:
-            return True
+        if project.version.tag:
+            return project.version.tag == on_disk_version.tag
+        if not project.version.revision or not on_disk_version.revision:
+            return False
         return (
-            project.version.tag == on_disk_version.tag
-            and project.version.revision == on_disk_version.revision
+            project.version.revision == on_disk_version.revision
             and self.revision_is_enough()
         )