make default branch configurable

Thanks! Thinking about this, I think we need to consider these use cases:

1. New repository with no remote. Probably want the current branch, as you're unlikely to be branching yet.
2. Self-owned repository where you work with just one remote (typically origin)
3. Contributing repository where you refer to a canonical fork (generally upstream or origin, depending on local convention)

I would probably use the following preference order:

1. git rev-parse --abbrev-ref upstream/HEAD
2. git rev-parse --abbrev-ref origin/HEAD
3. git rev-parse --abbrev-ref HEAD
4. git config init.defaultBranch
5. "master" (git too old to have init.defaultBranch)

This value should be computed on installation, not git_pieces_from_vcs(), and set in the configuration. We do VCS-specific installation steps in https://github.com/python-versioneer/python-versioneer/blob/master/src/git/install.py.

In git_pieces_from_vcs() we should use the configuration variable in place of hard-coded "master".

Now, this is just to get the default branch, which only needs to be done once at installation. When we actually use it, we should use the config variable, since that will be more reliable than each user of the repository naming their remotes the same thing.

We could consider making it default_branch = {default_branch}, which would make it easy to detect whether it has been set before or if we should run the inference above.

cool, will work on the above comment.