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