D2945: state: add a magicheader to each state file

[pulkit (Pulkit Goyal)]

pulkit added a subscriber: jeffpc.
pulkit added a comment.


  There is a suggestion from @jeffpc to have a plain text header in statefiles like `HGStateFile` so that user can grep for them or look into files and know that they are state files. How does that sound?
  (@jeffpc I am not sure I explained this correctly, it will be great if you can chime in.)

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D2945

To: pulkit, #hg-reviewers
Cc: jeffpc, yuja, mercurial-devel