index: expose rev property (#8909)

index: add rev property

This is very useful to identify indexes that are built from git revisions
and so are describing an immutable state and can be used for caching.

We were already using this for `__repr__`.

dvc/repo/index.py

@@ -128,10 +128,14 @@ class Index:
         self._params = params or {}
         self._collected_targets: Dict[int, List["StageInfo"]] = {}
 
-    def __repr__(self) -> str:
-        rev = "workspace"
+    @cached_property
+    def rev(self) -> Optional[str]:
         if not isinstance(self.repo.fs, LocalFileSystem):
-            rev = self.repo.get_rev()[:7]
+            return self.repo.get_rev()[:7]
+        return None
+
+    def __repr__(self) -> str:
+        rev = self.rev or "workspace"
         return f"Index({self.repo}, fs@{rev})"
 
     @classmethod