A leaner __git_ps1 function (~50%) for slow systems

example.sh

# Original __git_ps1
$ time orig__git_ps1 || 0
 (my-branch)
real    0m1.102s
user    0m0.000s
sys     0m0.140s

# Lean function
$ time __git_ps1 || 0
 (my-branch)
real    0m0.586s
user    0m0.000s
sys     0m0.077s

faster_git_ps1.sh

# Keep original function definition
eval orig"$(declare -f __git_ps1)"

function __git_ps1 {
  set -e
  branch="$(git rev-parse --abbrev-ref HEAD 2>/dev/null)"
  echo " ($branch)"
}