Include untracked files in dirtiness check

$PURE_GIT_UNTRACKED_DIRTY flag is used to determine the
inclusion of untracked files or lack thereof.

This commit changes the dirty-check command from
    git diff --quiet --ignore-submodules HEAD &>/dev/null
to
    git status --porcelain --ignore-submodules -uno
which is of fairly equal performance.
Update readme.md