add manpage for git bzr

[Karl-Philipp Richter]

It'd be nice if `man git bzr` would display the manpage of `git-bzr` (which eventually needs to be created) instead of the manpage of `git`.

ProblemType: Bug
DistroRelease: Ubuntu 15.10
Package: git-bzr 2:0.2+20140705-2
Uname: Linux 4.3.0-040300-generic x86_64
NonfreeKernelModules: nvidia
ApportVersion: 2.19.1-0ubuntu5
Architecture: amd64
CurrentDesktop: Unity
Date: Sat Jan 30 11:21:39 2016
InstallationDate: Installed on 2013-10-17 (834 days ago)
InstallationMedia: Ubuntu 13.04 "Raring Ringtail" - Release amd64 (20130424)
PackageArchitecture: all
SourcePackage: git-remote-bzr
UpgradeStatus: Upgraded to wily on 2015-11-19 (71 days ago)