Avoid calling heads() twice on every hg commit.

In an extreme case (merging two revisions with very low revision numbers)
this could be slower than the previous code, but it should be much faster
in the usual cases (parents are near the tip).  It also avoids some races
in some uninteresting cases (e.g. two concurrent hg commits).

mercurial/commands.py

@@ -548,9 +548,18 b' def commit(ui, repo, *pats, **opts):'
         return repo.commit(files, message, opts['user'], opts['date'], match,
                            force_editor=opts.get('force_editor'))
 
-    heads = repo.changelog.heads()
-    cmdutil.commit(ui, repo, commitfunc, pats, opts)
-    if len(repo.changelog.heads()) > len(heads):
+    node = cmdutil.commit(ui, repo, commitfunc, pats, opts)
+    if not node:
+        return
+    cl = repo.changelog
+    rev = cl.rev(node)
+    parents = cl.parentrevs(rev)
+    if rev - 1 in parents:
+        # one of the parents was the old tip
+        return
+    if (parents == (nullrev, nullrev) or
+        len(cl.heads(cl.node(parents[0]))) > 1 and
+        (parents[1] == nullrev or len(cl.heads(cl.node(parents[1]))) > 1)):
         ui.status(_('created new head\n'))
 
 def copy(ui, repo, *pats, **opts):