Use git rev-parse to obtain current branch name

Obtaining the current branch name using describe is roundabout and the
tokenization that includes '/' is broken for any branch with '/' in
its name (a common git practice). Using `git rev-parse --abbrev-ref
HEAD` seems to provide the current branch name in an easy to parse
fashion.
Merge pull request #22 from seth/sf/use-rev-parse

Use git rev-parse to obtain current branch name