update debrebuild fork to latest version from MR 212 from josch for src:debrebuild

 commit 4970c8bbf1692f2d69ba00b8a73ffde9659baeab (HEAD -> mr-origin-212)
 Author: Johannes 'josch' Schauer <josch@mister-muffin.de>
 Date:   Thu Dec 3 21:36:46 2020 +0100

    debrebuild: perform the actual build using sbuild or mmdebstrap as backends

     - Add the --builder option to select the rebuilder backend. The default is
       "none" which just prints instructions. Possible values are sbuild, dpkg,
       mmdebstrap, sbuild+unshare and pbuilder.
     - Add the --output option where the build results will be stored.
     - Compare the checksums from the input buildinfo with the build artifacts.
     - Unconditionally set Acquire::http::Dl-Limit "1000" and Acquire::Retries
       "5" to hopefully not get blocked by snapshot.d.o.
     - The sbuild+unshare as well as the mmdebstrap builders allow debrebuild
       to operate without any setup.  Closes: #958750
     - All builders (except the "none" builder) do download the source package
       automatically.  Closes: #961861
     - The "none" builder retains the former behavior of debrebuild. By using
       the other builders, debrebuild will actually perform the build itself
       and check if the produced build artifacts match.  Closes: #955123
     - All builders now support binNMUs.  Closes: #961862
     - Downgrades are avoided by using the last stable release.  Closes: #955307
     - There is now proper --help output and a man page.  Closes: #955049

Signed-off-by: Holger Levsen <holger@layer-acht.org>