Why would you want to recompile it? Well whatever.
First, grab them with: dget -u --insecure http://ftp.de.debian.org/debian/pool/main/m/mercurial/mercurial_1.5.2-1.dsc
Then run those commands. This will pack them into a deb like in the archive.
(I would just add a sid.list into my /etc/apt/sources.list.d/ and use apt-pinning since it grabs every needed dependencies, etc etc. You should check apt-pinning
out if you just want a newer version.)