stash: tell setup_revisions() to free our allocated strings

stash: tell setup_revisions() to free our allocated strings

In "git stash show", we do a first pass of parsing our command line
options by splitting them into revision args and stash args. These are
stored in strvecs, and we pass the revision args to setup_revisions().

But setup_revisions() may modify the argv we pass it, causing us to leak
some of the entries. In particular, if it sees a "--" string, that will
be dropped from argv. This is the same as other cases addressed by
f92dbdbc6a (revisions API: don't leak memory on argv elements that need
free()-ing, 2022-08-02), and we should fix it the same way: by passing
the free_removed_argv_elements option to setup_revisions().

I've added a test here which fails when built with SANITIZE=leak because
it calls "git stash show --". This by itself is not a very
interesting invocation, because there is nothing after the "--", and
thus the "--" is not really doing anything.

But I think the current parsing in show_stash() is a little
questionable. It splits the arguments into revision options and stash
options solely based on the presence of a leading dash, with no regard
to "--" at all. So:

  git stash show -- foo

will take "foo" as a stash option before we even pass anything to
setup_revisions(). And something like:

  git stash show -- 1

will show stash@{1}. But I would expect anything after the "--" to be a
pathspec. So in this example it would show only the part of the diff
that touched "foo". And something like:

  git stash show -p 1 -- foo

would treat "1" as a stash and "foo" as a pathspec.

That may be something we want to fix, but I want to focus here on the
leak-fixing without changing behavior. So this test is a little odd, but
does what we want without locking us in to any particular behavior (we
only care that "--" by itself does not change the output nor leak).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>