localrepo._tag: add a seek before writing the new tag

Some systems require a seek (or flush) between read and write
operations on the same file object.

This should fix issue838.

mercurial/localrepo.py

@@ -118,6 +118,7 b' class localrepository(repo.repository):'
         self.hook('pretag', throw=True, node=hex(node), tag=name, local=local)
 
         def writetag(fp, name, munge, prevtags):
+            fp.seek(0, 2)
             if prevtags and prevtags[-1] != '\n':
                 fp.write('\n')
             fp.write('%s %s\n' % (hex(node), munge and munge(name) or name))