This would be a fairly routine use of xstrfmt, except that
we need to remember the length of the result to pass to
cache_name_pos. So just use a strbuf, which makes this
simple.

As a bonus, this gets rid of confusing references to
"pathlen+1". The "1" is for the trailing slash we added, but
that is automatically accounted for in the strbuf's len
parameter.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>