Git prompt portion bug fixes

This patch fixes some bugs related to the git prompt portion of the
fish_right_prompt function. Namely, it fixes the following:

* A bug related to variable scoping. git_prompt was being defined, but
  as a block local variable that closed over the function's version. The
  -l flag has been removed.

* A bug related to empty git repos has been removed via judicious use of
  stderr piping to /dev/null.