Merge lp://staging/~sinzui/juju-release-tools/make-from-git into lp://staging/juju-release-tools

Add git support to the make-release-tarball.bash script.

This branch adds git support to the make-release-tarball.bash script.

The original args are preserved to build historic Lp:juju-core revnos and optionally revnos from another branch.
    make-release-tarball.bash -1
    make-release-tarball.bash 2825
    make-release-tarball.bash -1 lp:juju-core/1.18
    make-release-tarball.bash 1999 lp:juju-core/1.18

The git args are a branch name or revision id, and optionally a git repo. These inputs are valid
    make-release-tarball.bash master
    make-release-tarball.bash test
    make-release-tarball.bash 0ededa1f3e7cd8a50f1c94f6abbb3355735069a6
    make-release-tarball.bash test https://github.com/wallyworld/core.git
    make-release-tarball.bash 0ededa1f3e7cd8a50f1c94f6abbb3355735069a6 https://github.com/wallyworld/core.git

Note in my tests, wallyworld/core.git is not sane because it is setup as it's own package. It does not pretend to be juju/core.git or act as a fork that someone might make a pull request for.

The script sets up different vars to be clear about their uses. Only the means of getting the right tree are different. I chose a different strategy to build the tree with git that I hope avoids the problem of moved packages. I could make bzr to do the same, but it is now a legacy function. Once we have all the deps the deps are pinned and packages.