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

Historically "git stash [<options>]" was assumed to mean "git stash save
[<options>]". Since 1ada5020b3 (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, 9e140909f6 (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 8c3713cede
(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>

builtin/stash.c

@@ -1789,7 +1789,7 @@ static int push_stash(int argc, const char **argv, const char *prefix,
 	int ret;
 
 	if (argc) {
-		force_assume = !strcmp(argv[0], "-p");
+		force_assume = argc > 1 && !strcmp(argv[1], "-p");
 		argc = parse_options(argc, argv, prefix, options,
 				     push_assumed ? git_stash_usage :
 				     git_stash_push_usage,