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

@@ -259,6 +259,7 b' def postshare(sourcerepo, destrepo, book'
         fp.write("default = %s\n" % default)
         fp.close()
 
+    with destrepo.wlock():
     if bookmarks:
         fp = destrepo.vfs('shared', 'w')
         fp.write(sharedbookmarks + '\n')