fix: don't attempt to create redundant PR to start tracking obsolete …

…files (#755)

Check if the branch already exists.

autosynth/synth.py

@@ -376,7 +376,9 @@ def synthesize_loop(
     if not has_changes:
         if not toolbox.metadata_contains_generated_files(
             toolbox.branch
-        ) and toolbox.metadata_contains_generated_files(toolbox.sub_branch(youngest)):
+        ) and toolbox.metadata_contains_generated_files(
+            toolbox.sub_branch(youngest)
+        ) and not change_pusher.check_if_pr_already_exists(toolbox.branch):
             # Special case: the repo owner turned on obsolete file tracking.
             # Generate a one-time PR containing only metadata changes.
             executor.check_call(["git", "checkout", toolbox.branch])