Smarter defaults for --new-track

[mikepurvis]

Specifically:

• When the cwd is in a git repo, provide that git repo's remote as the suggestion for the upstream repo.
• When the cwd is in a git repo, provide the current branch as the suggestion for the upstream branch.
• Support an environment variable which allows parameterizing the default for the release repo, eg:

export BLOOM_DEFAULT_RELEASE_REPO="http://github.com/ros-gbp/@(upstream_repo_name)-release.git"

Obviously, --edit-track should retain the behaviour of suggesting as default the present configuration.

[wjwwood]

I'll put this on the long list of existing improvements. I'll hopefully get to them all in 0.5, so I'll add this to the 0.5 milestone.

[wjwwood]

Moving this to the untargeted milestone, doesn't look like I'll have time to implement it in this release.