allow git ubuntu shallow clone to speed up

[Christian Ehrhardt ]

There is one case left where I still prefer `pull-lp-source` to `git ubuntu clone`.
That is if I just quickly want to check content, because the former is faster.

But turning this around then I often enough need a different version so I need git-ubuntu again.

git itself has shallow clone/checkout which allows to be faster if you "only want to take a look".
I wanted to request that one could make a fast/shallow checkout with git-ubuntu as well.

Example:
 $ git ubuntu clone <pkg> <release>

would shallow-clone just pkg/<release>/ubuntu-devel

That is most of the time enough for what you want, and if it isn't it is just a `git fetch pkg` away then.

[Robie Basak (racb)]

I'm not sure if you do this already, or if it isn't suitable, but in some cases the Launchpad cgit view is a workaround for "just taking a look". For example: https://git.launchpad.net/ubuntu/+source/hello/tree/

But of course there are cases where having a shallow clone locally would be more convenient.

[Christian Ehrhardt  (paelzer)]

After yesterdays discussion it seems this might be more important (and to more people) than we thought. Maybe bump prio a bit on this one?

[Christian Ehrhardt  (paelzer)]

This will be even more useful once we add bug 1673710