commands: use dirstateguard instead of begin/end-parentchange for backout

Before this patch, "hg backout" uses 'begin'/'end'-'parentchange()'
of 'dirstate' class to avoid writing incomplete dirstate changes out
at failure.

But this framework doesn't work as expected, if 'dirstate.write()' is
invoked between them. In fact, in-memory dirstate changes may be
written out at 'repo.status()' implied by 'merge.update()', even
before this patch.

To restore dirstate as expected at failure of "hg backout", this patch
uses 'dirstateguard' instead of 'begin'/'end'-'parentchange()'.

mercurial/commands.py

@@ -547,14 +547,14 b' def backout(ui, repo, node=None, rev=Non'
         bheads = repo.branchheads(branch)
         rctx = scmutil.revsingle(repo, hex(parent))
         if not opts.get('merge') and op1 != node:
+            dsguard = cmdutil.dirstateguard(repo, 'backout')
             try:
                 ui.setconfig('ui', 'forcemerge', opts.get('tool', ''),
                              'backout')
-                repo.dirstate.beginparentchange()
                 stats = mergemod.update(repo, parent, True, True, False,
                                         node, False)
                 repo.setparents(op1, op2)
-                repo.dirstate.endparentchange()
+                dsguard.close()
                 hg._showstats(repo, stats)
                 if stats[3]:
                     repo.ui.status(_("use 'hg resolve' to retry unresolved "
@@ -567,6 +567,7 b' def backout(ui, repo, node=None, rev=Non'
                     return 0
             finally:
                 ui.setconfig('ui', 'forcemerge', '', '')
+                lockmod.release(dsguard)
         else:
             hg.clean(repo, node, show_stats=False)
             repo.dirstate.setbranch(branch)