stash: don't delete untracked files that match pathspec

stash: don't delete untracked files that match pathspec

Currently when 'git stash push -- <pathspec>' is used, untracked files
that match the pathspec will be deleted, even though they do not end up
in a stash anywhere.

This is because the original commit introducing the pathspec feature in
git stash push (df6bba0937 ("stash: teach 'push' (and 'create_stash') to
honor pathspec", 2017-02-28)) used the sequence of 'git reset <pathspec>
&& git ls-files --modified <pathspec> | git checkout-index && git clean
<pathspec>'.

The intention was to emulate what 'git reset --hard -- <pathspec>' would
do.  The call to 'git clean' was supposed to clean up the files that
were unstaged by 'git reset'.  This would work fine if the pathspec
doesn't match any files that were untracked before 'git stash push --
<pathspec>'.  However if <pathspec> matches a file that was untracked
before invoking the 'stash' command, all untracked files matching the
pathspec would inadvertently be deleted as well, even though they
wouldn't end up in the stash, and are therefore lost.

This behaviour was never what was intended, only blobs that also end up
in the stash should be reset to their state in HEAD, previously
untracked files should be left alone.

To achieve this, first match what's in the index and what's in the
working tree by adding all changes to the index, ask diff-index what
changed between HEAD and the current index, and then apply that patch in
reverse to get rid of the changes, which includes removal of added
files and resurrection of removed files.

Reported-by: Reid Price <reid.price@gmail.com>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>