xdl_merge(): move file1 and file2 labels to xmparam structure

xdl_merge(): move file1 and file2 labels to xmparam structure

The labels for the three participants in a potential conflict are all
optional arguments for the xdiff merge routine; if they are NULL, then
xdl_merge() can cope by omitting the labels from its output.  Move
them to the xmparam structure to allow new callers to save some
keystrokes where they are not needed.

This also has the virtue of making the xdiff merge interface more
similar to merge_trees, which might make it easier to learn.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>