GitLab

 - Need dot `.` for POSIX compliant instead of `source`
 - Use modern syntax for command substitution
 - Fix missing double quotes

Really this time :P

Fixes #4533

We need to use POSIX syntax. This file is no longer executed with zsh.

Fixes #4483.

Prompt the user before removing oh-my-zsh from the system

* Balk at incompatible Windows/MSYS git
* Test for chsh presence before trying to use it
* Replace non-portable `[[ ... ]]` and `[ x = *pattern* ]` constructs