Accelerate Bloom by using `git clone --depth 1`

[thomas-moulard]

Apparently, the last Git versions allow to commit and push on top of a shallow clone.
Given that the rosdistro repository is getting larger and larger, it could be a nice improvement.

At my location, cloning rosdistro (and my own package) is by far the longest operation.

Cf (for instance) http://stackoverflow.com/questions/6941889/is-git-clone-depth-1-shallow-clone-more-useful-than-it-makes-out

[note that this version of git is quite new so Bloom would need to make sure that the installed Git is recent enough, >=1.9-rc2]

[mikepurvis]

+1

[wjwwood]

Sounds like a useful feature, but I probably won't have time to implement it in the near future, therefore I'll set it to untargeted.