subsplit update does not work on repositories without a master branch

[tstoeckler]

Although developing in a master is a common practice among many projects, such a branch is not required to exist. the subsplit update command, however, hardcodes such a branch and fails if it doesn't exist.

I see three options to solve this problem:

1. Remove the checking out and resetting of the master branch. I'm not really sure what purpose it serves in the first place, but I'm probably missing something.
2. Make the branch to check out and reset configurable with a --branchoption (or similar).
3. Automatically figure out the branch to check out and reset by some arcane magic.

I'd love to hear your thoughts on this. Thanks!