[security] Fix for #310

[ogr3]

The approach is to wait with evaluation of the parts that contain the branch names until PS1 is rendered.

This patch handles both the ordinary branch name as well as the upstream branch name.

Try to fix #310.

Credits to @chmike and @jayrhynas

Steps to reproduce the bug and see if the fix works:

git clone https://github.com/njhartwell/pw3nage.git
cd pw3nage
GIT_PROMPT_SHOW_UPSTREAM=1
unset GIT_PROMPT_SHOW_UPSTREAM

Expected result when fixed without upstream info:

[$(./pw3n)|✔] ✔

With upstream info:

[$(./pw3n) {origin/$(./pw3n)}|✔] ✔

[ogr3]

Good catch - makes the patch even leaner.

[magicmonty]

Seems to work for me. Thanks @ogr3

[MPLew-is]

@guenhter your recent changes have un-done this fix - running the reproduction steps above again on the latest master results in arbitrary code execution!

[MPLew-is]

Looks like I was too hasty to assign blame, and running a git bisect shows it was @magicmonty that un-did this in 21063bc.