don't accept bogus N in `HEAD~N'

In a very shallow audit, I spotted code where overflow was not detected. But it's hardly critical.

Currently,

  git-diff HEAD HEAD

is equivalent to this

  git-diff HEAD HEAD~18446744073709551616   # aka 2^64

Exercising git-rev-parse directly, currently I get this:

  $ git-rev-parse --no-flags --sq HEAD~18446744073709551616
  '639ca5497279607665847f2e3a11064441a8f2a6'

It'd be better to produce a diagnostic and fail:

  $ ./git-rev-parse --no-flags --sq -- HEAD~18446744073709551616 > /dev/null
  fatal: ambiguous argument 'HEAD~18446744073709551616': unknown revision or filename

The code in question is in sha1_name.c (get_sha1_1):

               int num = 0;
               ...
               while (cp < name + len)
                       num = num * 10 + *cp++ - '0';

Looking at how to fix it, my first reflex was to replace that loop with this one:

		while (cp < name + len) {
			int tmp = num * 10 + *cp++ - '0';
			if (INT_MAX / 10 < num || tmp < num)
				return -1;
			num = tmp;
		}

But INT_MAX is used nowhere else, so I wonder if git avoids using it for some reason. At least `make check' gripes about __INT_MAX__. Anyhow, here's the patch I used. With it, git still passes `make test'.