strbuf_readlink(): support link targets that exceed 2*PATH_MAX

The `strbuf_readlink()` function refuses to read link targets that exceed 2*PATH_MAX (even if a sufficient size was specified by the caller). The reason that that limit is 2*PATH_MAX instead of PATH_MAX is that the symlink targets do not need to be normalized. After running `ln -s a/../a/../a/../a/../b c`, the target of the symlink `c` will not be normalized to `b` but instead be much longer. As such, symlink targets' lengths can far exceed PATH_MAX. They are frequently much longer than 2*PATH_MAX on Windows, which actually supports paths up to 32,767 characters, but sets PATH_MAX to 260 for backwards compatibility. For full details, see https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation Let's just hard-code the limit used by `strbuf_readlink()` to 32,767 and make it independent of the current platform's PATH_MAX. Based-on-a-patch-by: Karsten Blees <karsten.blees@gmail.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>