strbuf_readlink(): support link targets that exceed PATH_MAX

strbuf_readlink(): support link targets that exceed PATH_MAX

The `strbuf_readlink()` function refuses to read link targets that
exceed PATH_MAX (even if a sufficient size was specified by the caller).

As some platforms (*cough* Windows *cough*) support longer paths, remove
this restriction (similar to `strbuf_getcwd()`).

Signed-off-by: Karsten Blees <blees@dcon.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>