git-prompt: fix expansion of branch colour codes

git-prompt: fix expansion of branch colour codes

Because of the wrapping of the branch name variable $b, the colour codes
in the variable don't get applied, but are instead printed directly in
the output. Move the wrapping of $b to before colour codes are inserted
to correct this. Revert move of branch name colour codes in tests, as
the branch name is now coloured after the wrapping instead of before.

Signed-off-by: Joakim Petersen <joak-pet@online.no>
Signed-off-by: Junio C Hamano <gitster@pobox.com>