tryimportone: use dirstateguard instead of beginparentchange/endparentchange

To fix the issue that the recent (in memory) dirstate isn't visible to
external process (e.g. "precommit" hook), a subsequent patch makes
"localrepository.commit()" invoke "dirstate.write()" in it.

This change will make "beginparentchange()" and "endparentchange()" on
dirstate in "cmdutil.tryimportone()" meaningless, because:

  - "dirstate.write()" writes changed data into ".hg/dirstate", but

  - aborting between "beginparentchange()" and "endparentchange()"
    doesn't cause any restoring ".hg/dirstate"

    it just discards changes in memory.

This patch uses "dirstateguard" instead of "beginparentchange()" and
"endparentchange()" in "cmdutil.tryimportone()" to restore
".hg/dirstate" during a failure even if "dirstate.write()" is executed
before a failure.

This patch uses "lockmod.release(dsguard)" instead of
"dsguard.release()", because processing may be aborted before
assignment to "dsguard" , and the "if dsguard" examination for safety is
redundant.

mercurial/cmdutil.py

@@ -818,6 +818,7 b' def tryimportone(ui, repo, hunk, parents'
     msg = _('applied to working directory')
 
     rejects = False
+    dsguard = None
 
     try:
         cmdline_message = logmessage(ui, opts)
@@ -859,7 +860,7 b' def tryimportone(ui, repo, hunk, parents'
 
         n = None
         if update:
-            repo.dirstate.beginparentchange()
+            dsguard = dirstateguard(repo, 'tryimportone')
             if p1 != parents[0]:
                 updatefunc(repo, p1.node())
             if p2 != parents[1]:
@@ -899,7 +900,7 b' def tryimportone(ui, repo, hunk, parents'
                 n = repo.commit(message, opts.get('user') or user,
                                 opts.get('date') or date, match=m,
                                 editor=editor, force=partial)
-            repo.dirstate.endparentchange()
+            dsguard.close()
         else:
             if opts.get('exact') or opts.get('import_branch'):
                 branch = branch or 'default'
@@ -937,6 +938,7 b' def tryimportone(ui, repo, hunk, parents'
             msg = _('created %s') % short(n)
         return (msg, n, rejects)
     finally:
+        lockmod.release(dsguard)
         os.unlink(tmpname)
 
 def export(repo, revs, template='hg-%h.patch', fp=None, switch_parent=False,