Improved git_prompt_status speed by 3
Current :
  ./test.sh  5,88s user 16,30s system 57% cpu 38,531 total
  ./test.sh  6,01s user 15,99s system 56% cpu 39,125 total
New :
  ./test.sh  2,69s user 4,85s system 67% cpu 11,221 total
  ./test.sh  2,65s user 4,88s system 66% cpu 11,350 total