unique_path: fix unlikely heap overflow

unique_path: fix unlikely heap overflow

When merge-recursive creates a unique filename, it uses a
template like:

  path~branch_%d

where the final "_%d" is filled by an incrementing counter
until we find a unique name. We allocate 8 characters for
the counter, but there is no logic to limit the size of the
integer.

Of course, this is extremely unlikely, as you would need a
hundred million collisions to trigger the problem.  Even if
an attacker constructed a specialized repo, it is unlikely
that the victim would have the patience to run the merge.

However, we can make it trivially correct (and hopefully
more readable) by using a strbuf.

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