rev-parse: demonstrate overflow of N for "foo^N" and "foo~N"

If the number gets too high for an int, weird things may happen, as
signed overflows are undefined.  Add a test to show this; rev-parse
"sucessfully" interprets 100000000000000000000000000000000 to be the
same as 0, at least on x64 with GCC 9.2.1 and Clang 8.0.1, which is
obviously bogus.

Signed-off-by: René Scharfe <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: rscharfe

t/t1506-rev-parse-diagnosis.sh

@@ -215,4 +215,12 @@ test_expect_success 'arg before dashdash must be a revision (ambiguous)' '
 	test_cmp expect actual
 '
 
+test_expect_failure 'reject Nth parent if N is too high' '
+	test_must_fail git rev-parse HEAD^100000000000000000000000000000000
+'
+
+test_expect_failure 'reject Nth ancestor if N is too high' '
+	test_must_fail git rev-parse HEAD~100000000000000000000000000000000
+'
+
 test_done