git: reduce stat buffer storage overhead

git: reduce stat buffer storage overhead

The stat() array is a whopping 480 bytes (on x86-64, Perl 5.28),
while the new packed representation of two 64-bit doubles as a
scalar is "only" 56 bytes.  This can add up when there's many
inboxes.  Just use a string comparison on the packed
representation.

Some 32-bit Perl builds (IIRC OpenBSD) lack quad support, so
doubles were chosen for pack() portability.