stash: allow "git stash -p <pathspec>" to assume push again

stash: allow "git stash -p <pathspec>" to assume push again

Historically "git stash [<options>]" was assumed to mean "git stash save
[<options>]". Since 1ada5020b38 (stash: use stash_push for no verb form,
2017-02-28) it is assumed to mean "git stash push [<options>]". As the
push subcommand supports pathspecs, 9e140909f61 (stash: allow pathspecs
in the no verb form, 2017-02-28) allowed "git stash -p <pathspec>" to
mean "git stash push -p <pathspec>". This was broken in 8c3713cede7
(stash: eliminate crude option parsing, 2020-02-17) which failed to
account for "push" being added to the start of argv in cmd_stash()
before it calls push_stash() and kept looking in argv[0] for "-p" after
moving the code to push_stash().

Fix this by regression by checking argv[1] instead of argv[0] and add a
couple of tests to prevent future regressions.

Helped-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>