Tell multi-parent diff about core.symlinks.

Tell multi-parent diff about core.symlinks.

When core.symlinks is false, and a merge of symbolic links had conflicts,
the merge result is left as a file in the working directory. A decision
must be made whether the file is treated as a regular file or as a
symbolic link. This patch treats the file as a symbolic link only if
all merge parents were also symbolic links.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <junkio@cox.net>