show_ident_date: fix tz range check

Commit 1dca155fe3fa (log: handle integer overflow in
timestamps, 2014-02-24) tried to catch integer overflow
coming from strtol() on the timezone field by comparing against
LONG_MIN/LONG_MAX. However, the intermediate "tz" variable
is an "int", which means it can never be LONG_MAX on LP64
systems; we would truncate the output from strtol before the
comparison.

Clang's -Wtautological-constant-out-of-range-compare notices
this and rightly complains.

Let's instead store the result of strtol in a long, and then
compare it against INT_MIN/INT_MAX. This will catch overflow
from strtol, and also overflow when we pass the result as an
int to show_date.

Reported-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/commit-dates-parsing-fix' into next

* jk/commit-dates-parsing-fix:
  show_ident_date: fix tz range check