Failed stash-pop cleanup now invokes git clean with literal pathspecs for
stash-derived untracked paths. Filenames such as `:(glob)*` are valid POSIX
filenames and valid Git pathspec magic; passing them as ordinary pathspecs with
`-x` could delete unrelated ignored artifacts that were never stashed and are
not recoverable from the preserved stash.
Extend the fallback regression with a literal `:(glob)*` stash file and an
ignored `build.log` that must survive cleanup.
Fixes#4175
roboomp
e109883ee2