R1:a4bfed8415c7

Censor response bodies from Mercurial error messages

Summary:
Ref T6755. In Git and Subversion, running `git clone http://google.com/` or `svn checkout http://google.com/` does not echo the response body.

In Mercurial, it does. Censor it from the output of `hg pull` and `hg clone`. This prevents an attacker from:

- Creating a Mercurial remote repository with URI `http://10.0.0.1/secrets/`; and
- reading the secrets out of the error message after the clone fails.

Test Plan: Set a Mercurial remote…