Comment # 4 on bug 1129036 from Dr. Werner Fink

(In reply to Dominique Leuenberger from comment #0)
> Bash has been changed to take /bin/sh under control of update-alternatives.
> 
> somewhat a noble goal, but while building live isos at leats, resulted in
> errors, as there is an install cycle between bash (requiring
> updtae-alternatives in the post script) and update-alternatives (requiring
> /bin/sh to execute its post script, migrating data)
> 
> SO, unless u-a manages to run its post script as lua code without an
> external dependency on /bin/sh, there is no chance to get this /bin/sh
> update-alternative managed for the time being

AFAIK I've skip this and the current spec file looks like this

 %post -p /bin/bash
 %{_sbindir}/update-alternatives --quiet --force \
         --install %{_bindir}/sh sh %{_bindir}/bash 10100

 %preun -p /bin/bash
 if test "$1" = 0; then
         %{_sbindir}/update-alternatives --quiet --remove sh %{_bindir}/bash
 fi

that is no /bin/sh is linked with u-a but the link /bin/sh -> /usr/bin/sh is
part of the file list without u-a 

Beside this ... what is about BtrFS here ... is /var and /var/lib part of the
file system ... as u-a uses an database of its current configuration below
/var/lib/alternatives/