diff-index --cc shows a 3-way diff between HEAD, index and working tree.

This implements a 3-way diff between the HEAD commit, the state in the
index, and the working directory.  This is like the n-way diff for a
merge, and uses much of the same code.  It is invoked with the -c flag
to git-diff-index, which it already accepted and did nothing with.

Signed-off-by: Paul Mackerras <paulus@samba.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

diff-lib.c

@@ -213,6 +213,31 @@ static int show_modified(struct rev_info *revs,
 		return -1;
 	}
 
+	if (revs->combine_merges && !cached &&
+	    (hashcmp(sha1, old->sha1) || hashcmp(old->sha1, new->sha1))) {
+		struct combine_diff_path *p;
+		int pathlen = ce_namelen(new);
+
+		p = xmalloc(combine_diff_path_size(2, pathlen));
+		p->path = (char *) &p->parent[2];
+		p->next = NULL;
+		p->len = pathlen;
+		memcpy(p->path, new->name, pathlen);
+		p->path[pathlen] = 0;
+		p->mode = ntohl(mode);
+		hashclr(p->sha1);
+		memset(p->parent, 0, 2 * sizeof(struct combine_diff_parent));
+		p->parent[0].status = DIFF_STATUS_MODIFIED;
+		p->parent[0].mode = ntohl(new->ce_mode);
+		hashcpy(p->parent[0].sha1, new->sha1);
+		p->parent[1].status = DIFF_STATUS_MODIFIED;
+		p->parent[1].mode = ntohl(old->ce_mode);
+		hashcpy(p->parent[1].sha1, old->sha1);
+		show_combined_diff(p, 2, revs->dense_combined_merges, revs);
+		free(p);
+		return 0;
+	}
+
 	oldmode = old->ce_mode;
 	if (mode == oldmode && !hashcmp(sha1, old->sha1) &&
 	    !revs->diffopt.find_copies_harder)