Re: safe.directory: preliminary clean-up

> One wrong assumption was that the function might yield NULL when
> there is no interpolation.  This led to the use of an extra "check"
> variable, conditionally holding either the interpolated or the
> original string.  The assumption was with us since 8959555c
> (setup_git_directory(): add an owner check for the top-level
> directory, 2022-03-02) originally introduced the safe.directory
> feature.
> 
> Another wrong assumption was that the function might yield the same
> pointer as the input when there is no interpolation.  This led to a
> conditional free'ing of the interpolated copy, that the conditional
> never skipped, as we always received an allocated string.