gitfast: fix prompt

Certain themes need the 'clean' mark, and some people like to have the
'dirty' one, just like the original git_promp_info().

We probably shouldn't be modifying that function, there's no other clean
way to achieve our own status information, so let's try to emulate the
original as much as possible.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>