Use the NEWS merge driver if available

As described in https://wiki.php.net/vcs/gitworkflow#the_news_file it
is possible to setup a custom merge driver for NEWS so git doesnt try
to merge it.  This commit sets the default merge driver for the NEWS
file to the merge driver called NEWS.

Author: dsp

.gitattributes

@@ -21,4 +21,4 @@ sapi/nsapi/nsapi.c              ident
 sapi/continuity/capi.c          ident
 Zend/RFCs/002.txt               ident
 Zend/RFCs/003.txt               ident
-NEWS                            merge=ours
+NEWS                            merge=NEWS