graft: add test for reading old graftstate files with new mechanism
This tests the reading of old graftstate file using the new logic. The tests
shows that if user is in middle of a graft and then updates their mercurial to
the version where we have new graftstate format, we can still read the old graft
state format files correctly.
Differential Revision: https://phab.mercurial-scm.org/D2597
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).