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

mercurial/commands.py

@@ -54,6 +54,7 b' from . import ('
     rewriteutil,
     scmutil,
     server,
+    state as statemod,
     streamclone,
     tags as tagsmod,
     templatekw,
@@ -2216,17 +2217,16 b' def _dograft(ui, repo, *revs, **opts):'
                                      **pycompat.strkwargs(opts))
 
     cont = False
+    graftstate = statemod.cmdstate(repo, 'graftstate')
     if opts.get('continue'):
         cont = True
         if revs:
             raise error.Abort(_("can't specify --continue and revisions"))
         # read in unfinished revisions
-        try:
+        if graftstate.exists():
             nodes = _readgraftstate(repo)['nodes']
             revs = [repo[node].rev() for node in nodes]
-        except IOError as inst:
-            if inst.errno != errno.ENOENT:
-                raise
+        else:
             cmdutil.wrongtooltocontinue(repo, _('graft'))
     else:
         if not revs: