commands: make backout acquire locks before processing

Before this patch, "hg backout" executes below before acquisition of
wlock.

  - cmdutil.checkunfinished()
  - cmdutil.bailifchanged()
  - repo.dirstate.parents()

It may cause unintentional result, if another command runs parallelly
(see also issue4368).

In addition to it, "hg backout" refers changelog for purposes below
without acquisition of store lock (slock), and it may cause
unintentional result, if store is updated parallelly.

  - show and update to the revision by 'repo.changelog.tip()'

  - examine for "created new head" by 'repo.branchheads()' and
    'cmdutil.commitstatus()'

To avoid this issue, this patch makes "hg backout" acquire wlock and
slock before processing.

mercurial/commands.py

@@ -574,6 +574,15 b' def backout(ui, repo, node=None, rev=Non'
     Returns 0 on success, 1 if nothing to backout or there are unresolved
     files.
     '''
+    wlock = lock = None
+    try:
+        wlock = repo.wlock()
+        lock = repo.lock()
+        return _dobackout(ui, repo, node, rev, commit, **opts)
+    finally:
+        release(lock, wlock)
+
+def _dobackout(ui, repo, node=None, rev=None, commit=False, **opts):
     if rev and node:
         raise error.Abort(_("please specify just one revision"))
 
@@ -612,7 +621,6 b' def backout(ui, repo, node=None, rev=Non'
         parent = p1
 
     # the backout should appear on the same branch
-    wlock = repo.wlock()
     try:
         branch = repo.dirstate.branch()
         bheads = repo.branchheads(branch)
@@ -675,7 +683,9 b' def backout(ui, repo, node=None, rev=Non'
             finally:
                 ui.setconfig('ui', 'forcemerge', '', '')
     finally:
-        wlock.release()
+        # TODO: get rid of this meaningless try/finally enclosing.
+        # this is kept only to reduce changes in a patch.
+        pass
     return 0
 
 @command('bisect',