[PATCH] Add --track option to git clone

The following series adds a --track option to git clone. If the --track option is specified only the given remote branch will be received and checked out.

It tries to make the following usecase possible: Imagine you are working on a project that has 1.x and a 2.x branch. The project itself requires a complex setup (webserver, configuration files, etc). Setting up 1.x and 2.x branch requires a lot of work, but a developer needs to maintain both. He'll use the --track option to clone the 2.x branch into a directory and does the same with the 1.x branch, where he setup the project. He can use locally separate repositories while still being able to push to just one remote repository.

I'm aware that it's not possible to give more than one --track option. Implementing the possibility to specify multiple --track option would certainly a good improvment later, but would also require a lot more work as far as I understand the clone code.

Being able to specify just one --track option is a compromise of doing a small change and implementing this feature.