Add ability to prevent creation of branches with the same name but different case

We're having a lot of issues at the moment caused of case differences in branches. Basically, because Windows file systems do not differentiate between case, but the *nix ones do, quite a lot of the time people will end up pushing the same branch but with a differently cased name. This causes problems when Windows users then try and fetch as Git starts moaning that commits don't match etc.

As simple fix for this could be achieved using a pre-receive hook that effectively checked whether or not this push would create a new branch, and then to confirm that the name of the branch doesn't already exist but with a differently cased name.

Obviously this is only really relevant to people with a mix of different users on different operating systems, but it would resolve so many headaches for me!