shared: take wlock for writting the 'shared' file

I do not see a reason why this should not be covered by the wlock.

mercurial/hg.py

@@ -259,10 +259,11 b' def postshare(sourcerepo, destrepo, book'
         fp.write("default = %s\n" % default)
         fp.close()
 
-    if bookmarks:
-        fp = destrepo.vfs('shared', 'w')
-        fp.write(sharedbookmarks + '\n')
-        fp.close()
+    with destrepo.wlock():
+        if bookmarks:
+            fp = destrepo.vfs('shared', 'w')
+            fp.write(sharedbookmarks + '\n')
+            fp.close()
 
 def _postshareupdate(repo, update, checkout=None):
     """Maybe perform a working directory update after a shared repo is created.