Revert _revision_only_mode: restore branch guard

The branch guard (not self.wanted_version.branch) is intentional.
When a project specifies both revision and branch, _check_for_newer_version
must follow the branch-aware path so the available version is reported as
'branch - revision'.  Removing it caused the branch prefix to be stripped
from the available output, breaking six BDD scenarios.

https://claude.ai/code/session_01Doq8oQtBRH4afusvp9Dv4p

Author: claude

dfetch/project/subproject.py

@@ -374,7 +374,11 @@ def _on_disk_hash(self) -> str | None:
 
     def _revision_only_mode(self) -> bool:
         """Return True when the wanted version should be resolved by revision alone."""
-        return bool(self.wanted_version.revision) and self.revision_is_enough()
+        return (
+            not self.wanted_version.branch
+            and bool(self.wanted_version.revision)
+            and self.revision_is_enough()
+        )
 
     def _check_for_newer_version(self) -> Version | None:
         """Check if a newer version is available on the given branch.