xdiff-merge: optionally show conflicts in "diff3 -m" style

When showing conflicting merges, we traditionally followed RCS's merge
output format.  The output shows:

 <<<<<<<
 postimage from one side;
 =======
 postimage of the other side; and
 >>>>>>>

Some poeple find it easier to be able to understand what is going on when
they can view the common ancestor's version, which is used by "diff3 -m",
which shows:

 <<<<<<<
 postimage from one side;
 |||||||
 shared preimage;
 =======
 postimage of the other side; and
 >>>>>>>

This is an initial step to bring that as an optional feature to git.
Only "git merge-file" has been converted, with "--diff3" option.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/xdiff/xdiff.h b/xdiff/xdiff.h
index 413082e..deebe02 100644
--- a/xdiff/xdiff.h
+++ b/xdiff/xdiff.h
@@ -50,10 +50,16 @@
 #define XDL_BDOP_CPY 2
 #define XDL_BDOP_INSB 3
 
+/* merge simplification levels */
 #define XDL_MERGE_MINIMAL 0
 #define XDL_MERGE_EAGER 1
 #define XDL_MERGE_ZEALOUS 2
 #define XDL_MERGE_ZEALOUS_ALNUM 3
+#define XDL_MERGE_LEVEL_MASK 0x0f
+
+/* merge output styles */
+#define XDL_MERGE_DIFF3 0x8000
+#define XDL_MERGE_STYLE_MASK 0x8000
 
 typedef struct s_mmfile {
 	char *ptr;