fixup! Win32: fix checkout problem with directories exceeding MAX_PATH

We cannot magically enlarge cache_def's PATH_MAX-sized buffer...
fixup! Win32: fix checkout problem with directories exceeding MAX_PATH

We must not use MAX_LONG_PATH outside of the Windows-specific code...
Use a strbuf in symlink

This also happens to get rid of the length limitation imposed on
Windows (we cannot use MAX_LONG_PATH here because that is only defined
in the Windows-specific part of compat/).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>