state: write the version number in plain text on top of state files We will soon be using CBOR format to write the data in state files. But we should not write the version number of the state files in CBOR format and we should rather write it in plain text because in future we can change the format of state files and we should be able to parse the version number of state file without requiring to understand a certain format. This will help us in making sure we have a good compatibility story with other versions of state files. Differential Revision: https://phab.mercurial-scm.org/D3579