convert: avoid updating tags when there is nothing new

Previously, when converting from a mercurial repo there would be an extraneous
commit at the end of the convert process that would rewrite tags. Now, we check
if there are any new tags before doing this rewriting.

hgext/convert/hg.py

@@ -212,6 +212,25 b' class mercurial_sink(converter_sink):'
         newlines = sorted([("%s %s\n" % (tags[tag], tag)) for tag in tags])
         if newlines == oldlines:
             return None, None
+
+        # if the old and new tags match, then there is nothing to update
+        oldtags = set()
+        newtags = set()
+        for line in oldlines:
+            s = line.strip().split(' ', 1)
+            if len(s) != 2:
+                continue
+            oldtags.add(s[1])
+        for line in newlines:
+            s = line.strip().split(' ', 1)
+            if len(s) != 2:
+                continue
+            if s[1] not in oldtags:
+                newtags.add(s[1].strip())
+
+        if not newtags:
+            return None, None
+
         data = "".join(newlines)
         def getfilectx(repo, memctx, f):
             return context.memfilectx(f, data, False, False, None)