gpg: make sign acquire wlock before processing

Before this patch, "hg sign" of gpg extension executes/evaluates below
without acquisition of wlock.

  - repo.dirstate.parents()
  - '.hgsigs' not in repo.dirstate

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

To avoid this issue, this patch makes "hg sign" of gpg extension
acquire wlock before processing.

hgext/gpg.py

@@ -9,6 +9,7 b' import os, tempfile, binascii'
 from mercurial import util, commands, match, cmdutil, error
 from mercurial import node as hgnode
 from mercurial.i18n import _
+from mercurial import lock as lockmod
 
 cmdtable = {}
 command = cmdutil.command(cmdtable)
@@ -222,7 +223,14 b' def sign(ui, repo, *revs, **opts):'
 
     See :hg:`help dates` for a list of formats valid for -d/--date.
     """
+    wlock = None
+    try:
+        wlock = repo.wlock()
+        return _dosign(ui, repo, *revs, **opts)
+    finally:
+        lockmod.release(wlock)
 
+def _dosign(ui, repo, *revs, **opts):
     mygpg = newgpg(ui, **opts)
     sigver = "0"
     sigmessage = ""