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

graft: start using the cmdstate class to read and write data to graftstate This patch replaces the logic to read and write data to graftstate file to use the state.cmdstate() class. The previous graftstate format didn't had any version number on top of that, so we have to catch the CorruptedState error and then read the graftstate in case of old state files. This will help us to implement nice additions to graft commands like `--no-commit`, `--abort`, `--stop` flags. Passing on test-graft.t shows that things are working fine. Differential Revision: https://phab.mercurial-scm.org/D3654

graft: use state.cmdstate() to check whether graftstate exists This is a step towards make graft use the new state.cmdstate() class. This patch replaces the ugly try-except with nice if-else conditionals. Differential Revision: https://phab.mercurial-scm.org/D3652