upload-pack: fix ambiguous error message

upload-pack.c takes any --shallow-exclude argument(s) from
clone/fetch/etc. and passes them through expand_ref().  If it does not
get back exactly one ref from the call to expand_ref(), it will die with
the following error:

    fatal: git upload-pack: ambiguous deepen-not: %s

Given that the documentation suggests to users that --shallow-exclude
accepts a revision rather than a ref (which will be corrected in a
subsequent commit), users may try to pass a revision.  In such a case,
expand_ref() will return 0 matches, but the error message we print will
be misleading since "ambiguous" suggests there are multiple matches.
Provide a clearer error message for such a case.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: newren

t/t5500-fetch-pack.sh

@@ -926,6 +926,13 @@ test_expect_success 'fetch exclude tag one' '
 	test_cmp expected actual
 '
 
+test_expect_success 'fetch exclude tag one as revision' '
+	test_when_finished rm -f rev err &&
+	git -C shallow-exclude rev-parse one >rev &&
+	test_must_fail git -C shallow12 fetch --shallow-exclude $(cat rev) origin 2>err &&
+	grep "deepen-not is not a ref:" err
+'
+
 test_expect_success 'fetching deepen' '
 	test_create_repo shallow-deepen &&
 	(

upload-pack.c

@@ -1025,10 +1025,14 @@ static int process_deepen_not(const char *line, struct oidset *deepen_not, int *
 {
 	const char *arg;
 	if (skip_prefix(line, "deepen-not ", &arg)) {
+		int cnt;
 		char *ref = NULL;
 		struct object_id oid;
-		if (expand_ref(the_repository, arg, strlen(arg), &oid, &ref) != 1)
+		cnt = expand_ref(the_repository, arg, strlen(arg), &oid, &ref);
+		if (cnt > 1)
 			die("git upload-pack: ambiguous deepen-not: %s", line);
+		if (cnt < 1)
+			die("git upload-pack: deepen-not is not a ref: %s", line);
 		oidset_insert(deepen_not, &oid);
 		free(ref);
 		*deepen_rev_list = 1;