There are multiple approaches to "rewriting history" in a dvcs depending on what your workflow and preferences are.

In Hg you can do various forms of rebasing and rewriting history, e.g.: * http://mercurial.selenic.com/wiki/ConcatenatingChangesets * http://mercurial.selenic.com/wiki/EditingHistory * http://mercurial.selenic.com/wiki/RebaseExtension

The difference between Hg and Git in this regard is much more a question of what's the basis of their philosophy. Git promotes rewriting & rebasing while Hg is biased towards a more conservative approach.