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

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 <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>