Rename Repository.head to Repository.head_ref

This is to avoid confusion with the Stack.head function which returns
a commit object rather than a file name. The patch also changes the
"new" function to use stack.head directly rather than via the
Repository.refs... object.

Signed-off-by: Catalin Marinas <catalin.marinas@gmail.com>
Acked-by: Karl Hasselström <kha@treskal.com>