fsck: avoid parse_timestamp() on buffer that isn't NUL-terminated

fsck: avoid parse_timestamp() on buffer that isn't NUL-terminated

In fsck_ident(), we parse the timestamp with parse_timestamp(), which is
really an alias for strtoumax(). But since our buffer may not be
NUL-terminated, this can trigger a complaint from ASan's
strict_string_checks mode. This is a false positive, since we know that
the buffer contains a trailing newline (which we checked earlier in the
function), and that strtoumax() would stop there.

But it is worth working around ASan's complaint. One is because that
will let us turn on strict_string_checks by default, which has helped
catch other real problems. And two is that the safety of the current
code is very hard to reason about (it subtly depends on distant code
which could change).

One option here is to just parse the number left-to-right ourselves. But
we care about the size of a timestamp_t and detecting overflow, since
that's part of the point of these checks. And doing that correctly is
tricky. So we'll instead just pull the digits into a separate,
NUL-terminated buffer, and use that to call parse_timestamp().

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>