command: Move smart sync override logic to Command base class

Deduplicate the logic for loading `smart_sync_override.xml` by
moving it from `forall.py` to the `Command` base class. This allows
other commands to reuse it to respect smart tags.

Bug: 279204331
Change-Id: I6f2f03995266c2a68c3225cacb92b2f580a89178
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/582502
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Mike Frysinger <vapier@google.com>

command.py

@@ -516,6 +516,22 @@ class Command:
         )
         return result
 
+    def GetSmartSyncOverridePath(self, manifest=None) -> str:
+        """Return the path where smart sync writes its override manifest."""
+        if manifest is None:
+            manifest = self.manifest
+        return os.path.join(
+            manifest.manifestProject.worktree, "smart_sync_override.xml"
+        )
+
+    def TryOverrideManifestWithSmartSync(self, manifest=None) -> None:
+        """Override manifest with smart_sync_override.xml if it exists."""
+        if manifest is None:
+            manifest = self.manifest
+        smart_sync_manifest_path = self.GetSmartSyncOverridePath(manifest)
+        if os.path.isfile(smart_sync_manifest_path):
+            manifest.Override(smart_sync_manifest_path)
+
     def ManifestList(self, opt):
         """Yields all of the manifests to traverse.