Parse git branch info when running in developer checkout

[smarr]

Currently, we simply do result['branchOrTag'] = _exec(git_cmd + ['show', '-s', '--format=%D', 'HEAD']), which results in things like HEAD -> develop, origin/master, origin/develop, origin/HEAD, master on a local checkout as used for development.

This isn't particularly helpful in most cases.

We'd probably get good enough results by taking what ever comes after HEAD -> and drop the bits after the ,.