Merge branch 'rh/prompt-pcmode-avoid-eval-on-refname' into maint

The shell prompt script (in contrib/), when using the PROMPT_COMMAND
interface, used an unsafe construct when showing the branch name in
$PS1.

* rh/prompt-pcmode-avoid-eval-on-refname:
git-prompt.sh: don't put unsanitized branch names in $PS1