log-tree: use FLEX_ARRAY in name_decoration

We are already using the flex-array technique; let's
annotate it with our usual FLEX_ARRAY macro. Besides being
more readable, this is slightly more efficient on compilers
that understand flex-arrays.

Note that we need to bump the allocation in add_name_decoration,
which did not explicitly add one byte for the NUL terminator
of the string we are putting into the flex-array (it did not
need to before, because the struct itself was over-allocated
by one byte).

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