Fix Create branch function to disallow branch that starts or end with '/'

[StanleyGoldman]

If the branch name starts or ends with "/" it is invalid.

[thorade]

There are some more forbidden characters, e.g. any of ~ ^ : \ and some more!
https://stackoverflow.com/questions/3651860/which-characters-are-illegal-within-a-branch-name

[StanleyGoldman]

I'm pretty sure we got most of that covered. Here in Utility.cs#L31, please feel free to double check me.

What we aren't checking for is specifically starting or ending with /