Merge branch 'so/prompt-command'

Updates __git_ps1 so that it can be used as $PROMPT_COMMAND,
instead of being used for command substitution in $PS1, to embed
color escape sequences in its output.

* so/prompt-command:
  coloured git-prompt: paint detached HEAD marker in red
  Fix up colored git-prompt
  show color hints based on state of the git tree
  Allow __git_ps1 to be used in PROMPT_COMMAND