convert-repo: avoid adding bogus value to shamap on tag update

contrib/convert-repo

@@ -295,7 +295,8 b' class convert:'
             nrev = self.dest.puttags(ctags)
             # write another hash correspondence to override the previous
             # one so we don't end up with extra tag heads
-            file(self.mapfile, "a").write("%s %s\n" % (c, nrev))
+            if nrev:
+                file(self.mapfile, "a").write("%s %s\n" % (c, nrev))
 
 gitpath, hgpath, mapfile = sys.argv[1:]
 if os.path.isdir(gitpath + "/.git"):