fast-export: use xmemdupz() for anonymizing oids

Our anonymize_mem() function is careful to take a ptr/len pair to allow
storing binary tokens like object ids, as well as partial strings (e.g.,
just "foo" of "foo/bar"). But it duplicates the hash key using
xstrdup()! That means that:

  - for a partial string, we'd store all bytes up to the NUL, even
    though we'd never look at anything past "len". This didn't produce
    wrong behavior, but was wasteful.

  - for a binary oid that doesn't contain a zero byte, we'd copy garbage
    bytes off the end of the array (though as long as nothing complained
    about reading uninitialized bytes, further reads would be limited by
    "len", and we'd produce the correct results)

  - for a binary oid that does contain a zero byte, we'd copy _fewer_
    bytes than intended into the hashmap struct. When we later try to
    look up a value, we'd access uninitialized memory and potentially
    falsely claim that a particular oid is not present.

The most common reason to store an oid is an anonymized gitlink, but our
test case doesn't have any gitlinks at all. So let's add one whose oid
contains a NUL and is present at two different paths. ASan catches the
memory error, but even without it we can detect the bug because the oid
is not anonymized the same way for both paths.

And of course the fix is to copy the correct number of bytes. We don't
technically need the appended NUL from xmemdupz(), but it doesn't hurt
as an extra protection against anybody treating it like a string (plus a
future patch will push us more in that direction).

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

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