added support for bazaar in agnoster theme (#5016)

the agnoster theme is now able to render basic information
if the user is currently residing inside a bazaar folder.
if so, it will render a green promt segment with "bzr" and
the current revision number in it. if there are untracked
files, the bar will be rendered in yellow. if there are
changes to already tracked files, a + character will be
printed.