reconstruct state data

If the state file is missing, we can reconstruct which parts were
exported based on revision ids

Also, packages could have branches in Git, but not be in Git. We
need to check (project,package) tuple for this and not just abort
based on the package name alone.

lib/git.py

@@ -160,6 +160,12 @@ class Git:
             .strip()
         )
 
+    def branch_commit(self, branch="HEAD"):
+      try:
+        return (self.git_run(["cat-file", "commit", branch], stdout=subprocess.PIPE).stdout.decode("utf-8").strip())
+      except:
+        return ''
+
     def set_branch_head(self, branch, commit):
         return self.git_run(["update-ref", f"refs/heads/{branch}", commit])