diffstat: do not rely on uninitialized data

Right now if you visit:
<http://git.zx2c4.com/systemd/diff/src/udev/udev-builtin-input_id.c?id=bcfce235>
you'll see that if you reload the page a few times, a bunch of times the
diffstat comes out with no lines being shown or changed. I'm not
currently sure what the cause of this is, but I suspect it might have to
do with this uninitialized data.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Add a cache-snapshot-ttl configuration variable

This can be used to specify the TTL for snapshots. Snapshots are usually
static and do not ever change. On the other hand, tarball generation is
CPU intensive.

One use case of this setting (apart from increasing the lifetime of
snapshot cache slots) is caching of snapshots while disabling the cache
for static/dynamic HTML pages (by setting TTL to zero for everything
except for snapshot requests).

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>