Do not fetch all branches of remote `requires`

[jayvdb]

When requires: are used, all branches are fetched.

This can likely be solved with a simple CLI arg when invoking subprocess git.

Note that switching to using GitPython (#169) would also likely solve this as it wouldnt have the same defaults as the cli git.

[jayvdb]

(Does the current git integration allow branches to be specified in the requires: ? c.f. #75)

[chfw]

need to change code to squeeze in a bit parameters.

[chfw]

wondered if GitPython become integrated, this need could be coded in that code change.

[jayvdb]

Yes, switching to GitPython would make all of the git features easier, with less side effects.