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
#require unix-permissions no-root
$ hg init a
$ cd a
$ echo foo > b
$ hg add b
$ hg ci -m "b"
$ chmod -w .hg/store
$ cd ..
$ hg clone a b
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 97310831fa1a
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ chmod +w a/.hg/store # let test clean up
$ cd b
$ hg verify
checking changesets
checking manifests
crosschecking files in changesets and manifests
checking files
1 files, 1 changesets, 1 total revisions
$ cd ..