Reference a submodule branch instead of a commit

[Jeremy Morton]

At the moment, supermodules must reference a given commit in each of 
its submodules.  If one is in control of a submodule and it changes on 
a regular basis, this can cause a lot of overhead with "submodule 
updated" commits in the supermodule.  It would be useful of git allows 
the option of referencing a submodule's branch instead of a given 
submodule commit.  How about adding this functionality?

--
Best regards,
Jeremy Morton (Jez)