Merge branch 'ah/git-prompt-portability'

The command line prompt support used to be littered with bash-isms,
which has been corrected to work with more shells.

* ah/git-prompt-portability:
  git-prompt: support custom 0-width PS1 markers
  git-prompt: ta-da! document usage in other shells
  git-prompt: don't use shell $'...'
  git-prompt: add some missing quotes
  git-prompt: replace [[...]] with standard code
  git-prompt: don't use shell arrays
  git-prompt: fix uninitialized variable
  git-prompt: use here-doc instead of here-string