reftable tests: avoid "int" overflow, use "uint64_t"

Change code added in 1ae2b8cda8 (reftable: add merged table view,
2021-10-07) to consistently use the "uint64_t" type. These "min" and
"max" variables get passed in the body of this function to a function
whose prototype is:

    [...] reftable_writer_set_limits([...], uint64_t min, uint64_t max

This avoids the following warning on SunCC 12.5 on
gcc211.fsffrance.org:

    "reftable/merged_test.c", line 27: warning: initializer does not fit or is out of range: 0xffffffff

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

reftable/merged_test.c

@@ -24,8 +24,8 @@ https://developers.google.com/open-source/licenses/bsd
 static void write_test_table(struct strbuf *buf,
 			     struct reftable_ref_record refs[], int n)
 {
-	int min = 0xffffffff;
-	int max = 0;
+	uint64_t min = 0xffffffff;
+	uint64_t max = 0;
 	int i = 0;
 	int err;