tests/test-issue612.t

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

https://bz.mercurial-scm.org/612

  $ hg init
  $ mkdir src
  $ echo a > src/a.c
  $ hg ci -Ama
  adding src/a.c

  $ hg mv src source
  moving src/a.c to source/a.c

  $ hg ci -Ammove

  $ hg co -C 0
  1 files updated, 0 files merged, 1 files removed, 0 files unresolved

  $ echo new > src/a.c
  $ echo compiled > src/a.o
  $ hg ci -mupdate
  created new head

  $ hg status
  ? src/a.o

  $ hg merge
  merging src/a.c and source/a.c to source/a.c
  0 files updated, 1 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)

  $ hg status
  M source/a.c
  R src/a.c
  ? src/a.o