stash push -u: don't create empty stash

stash push -u: don't create empty stash

When introducing the stash push feature, and thus allowing users to pass
in a pathspec to limit the files that would get stashed in
df6bba0937 ("stash: teach 'push' (and 'create_stash') to honor
pathspec", 2017-02-28), this developer missed one place where the
pathspec should be passed in.

Namely in the call to the 'untracked_files()' function in the
'no_changes()' function.  This resulted in 'git stash push -u --
<non-existant>' creating an empty stash when there are untracked files
in the repository other that don't match the pathspec.

As 'git stash' never creates empty stashes, this behaviour is wrong and
confusing for users.  Instead it should just show a message "No local
changes to save", and not create a stash.

Luckily the 'untracked_files()' function already correctly respects
pathspecs that are passed to it, so the fix is simply to pass the
pathspec along to the function.

Reported-by: Marc Strapetz <marc.strapetz@syntevo.com>
Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>