Description:
When package is installed, it always force links /bin/sh to --> /bin/bash .
Some systems use Zsh or some other shell like busybox to provide /bin/sh .
While it is important to ensure /bin/sh is present and working properly, the install
script should check if sh is already present and attached to another known shell
as found in "core" or "extra" repositories, and if it is, then not write over
the symlink/file. Actually all the shells that provide /bin/sh should do this,
so they all can work in an expected manner and exist along side each other.
If relavant versions of a symlink could be tracked, sh could be installed
as sh.pacnew and that would solve it -- I am not aware of that being possible, though.

Additional info:
zsh 5.2-1
bash 4.3.042-4



Steps to reproduce:

Install zsh, manually symlink /bin/sh --> /bin/zsh, install bash. /bin/sh changed!