stash: fix show referencing stash index

In the conversion of 'stash show' to C in dc7bd382 ("stash: convert
show to builtin", 2019-02-25), 'git stash show <n>', where n is the
index of a stash got broken, if n is not a file or a valid revision by
itself.

'stash show' accepts any flag 'git diff' accepts for changing the
output format.  Internally we use 'setup_revisions()' to parse these
command line flags.  Currently we pass the whole argv through to
'setup_revisions()', which includes the stash index.

As the stash index is not a valid revision or a file in the working
tree in most cases however, this 'setup_revisions()' call (and thus
the whole command) ends up failing if we use this form of 'git stash
show'.

Instead of passing the whole argv to 'setup_revisions()', only pass
the flags (and the command name) through, while excluding the stash
reference.  The stash reference is parsed (and validated) in
'get_stash_info()' already.

This separate parsing also means that we currently do produce the
correct output if the command succeeds.
Reported-by: NMike Hommey <mh@glandium.org>
Signed-off-by: NThomas Gummerer <t.gummerer@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/stash.c

@@ -713,11 +713,11 @@ static int git_stash_config(const char *var, const char *value, void *cb)
 static int show_stash(int argc, const char **argv, const char *prefix)
 {
 	int i;
-	int opts = 0;
 	int ret = 0;
 	struct stash_info info;
 	struct rev_info rev;
 	struct argv_array stash_args = ARGV_ARRAY_INIT;
+	struct argv_array revision_args = ARGV_ARRAY_INIT;
 	struct option options[] = {
 		OPT_END()
 	};
@@ -726,11 +726,12 @@ static int show_stash(int argc, const char **argv, const char *prefix)
 	git_config(git_diff_ui_config, NULL);
 	init_revisions(&rev, prefix);
 
+	argv_array_push(&revision_args, argv[0]);
 	for (i = 1; i < argc; i++) {
 		if (argv[i][0] != '-')
 			argv_array_push(&stash_args, argv[i]);
 		else
-			opts++;
+			argv_array_push(&revision_args, argv[i]);
 	}
 
 	ret = get_stash_info(&info, stash_args.argc, stash_args.argv);
@@ -742,7 +743,7 @@ static int show_stash(int argc, const char **argv, const char *prefix)
 	 * The config settings are applied only if there are not passed
 	 * any options.
 	 */
-	if (!opts) {
+	if (revision_args.argc == 1) {
 		git_config(git_stash_config, NULL);
 		if (show_stat)
 			rev.diffopt.output_format = DIFF_FORMAT_DIFFSTAT;
@@ -756,7 +757,7 @@ static int show_stash(int argc, const char **argv, const char *prefix)
 		}
 	}
 
-	argc = setup_revisions(argc, argv, &rev, NULL);
+	argc = setup_revisions(revision_args.argc, revision_args.argv, &rev, NULL);
 	if (argc > 1) {
 		free_stash_info(&info);
 		usage_with_options(git_stash_show_usage, options);

t/t3903-stash.sh

@@ -708,6 +708,24 @@ test_expect_success 'invalid ref of the form "n", n >= N' '
 	git stash drop
 '
 
+test_expect_success 'valid ref of the form "n", n < N' '
+	git stash clear &&
+	echo bar5 >file &&
+	echo bar6 >file2 &&
+	git add file2 &&
+	git stash &&
+	git stash show 0 &&
+	git stash branch tmp 0 &&
+	git checkout master &&
+	git stash &&
+	git stash apply 0 &&
+	git reset --hard &&
+	git stash pop 0 &&
+	git stash &&
+	git stash drop 0 &&
+	test_must_fail git stash drop
+'
+
 test_expect_success 'branch: do not drop the stash if the branch exists' '
 	git stash clear &&
 	echo foo >file &&