support using branches with name other than 'pristine-tar'

[Scott Moser]

git-ubuntu is storing pristine tar information in its branches.
The branch name is not 'pristine-tar' (and may not be local).

It seems useful to allow the caller of pristine-tar to supply the reference branch.

Ie,
  pristine-tar --branch=my-pristine-tar list
  pristine-tar --branch=my-pristine-tar checkout <TARBALL>

ProblemType: Bug
DistroRelease: Ubuntu 17.10
Package: pristine-tar 1.40
ProcVersionSignature: Ubuntu 4.10.0-26.30-generic 4.10.17
Uname: Linux 4.10.0-26-generic x86_64
NonfreeKernelModules: zfs zunicode zavl zcommon znvpair
ApportVersion: 2.20.6-0ubuntu4
Architecture: amd64
CurrentDesktop: GNOME
Date: Wed Aug 2 13:06:46 2017
EcryptfsInUse: Yes
InstallationDate: Installed on 2015-07-23 (741 days ago)
InstallationMedia: Ubuntu 15.10 "Wily Werewolf" - Alpha amd64 (20150722.1)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: pristine-tar
UpgradeStatus: No upgrade log present (probably fresh install)

[Nish Aravamudan (nacc)]

To be clear, this won't really help the importer. We don't use pristine-tar directly (and in fact, you can't in the most general case), but have to use gbp to extract orig tarballs, because of component tarballs. So, really, it would be most useful to have this flag to gbp as well, which probably relies on pristine-tar having this :)