blame: Notice a wholesale incorporation of an existing file.

The -C option to blame tries to find a section of a preimage
file by running diff against the lines whose origin is still
unknown, and excluding the different parts.  The code however
did not cover the case where the tail part of the section
matched, which we handle for the normal non-move/copy codepath.

This breakage was most visible when preimage file matches in its
entirety and failed to pass blame in such a case.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-blame.c

@@ -890,6 +890,39 @@ static void copy_split_if_better(struct scoreboard *sb,
 	memcpy(best_so_far, this, sizeof(struct blame_entry [3]));
 }
 
+/*
+ * We are looking at a part of the final image represented by
+ * ent (tlno and same are offset by ent->s_lno).
+ * tlno is where we are looking at in the final image.
+ * up to (but not including) same match preimage.
+ * plno is where we are looking at in the preimage.
+ *
+ * <-------------- final image ---------------------->
+ *       <------ent------>
+ *         ^tlno ^same
+ *    <---------preimage----->
+ *         ^plno
+ *
+ * All line numbers are 0-based.
+ */
+static void handle_split(struct scoreboard *sb,
+			 struct blame_entry *ent,
+			 int tlno, int plno, int same,
+			 struct origin *parent,
+			 struct blame_entry *split)
+{
+	if (ent->num_lines <= tlno)
+		return;
+	if (tlno < same) {
+		struct blame_entry this[3];
+		tlno += ent->s_lno;
+		same += ent->s_lno;
+		split_overlap(this, ent, tlno, plno, same, parent);
+		copy_split_if_better(sb, split, this);
+		decref_split(this);
+	}
+}
+
 /*
  * Find the lines from parent that are the same as ent so that
  * we can pass blames to it.  file_p has the blob contents for
@@ -922,26 +955,21 @@ static void find_copy_in_blob(struct scoreboard *sb,
 
 	patch = compare_buffer(file_p, &file_o, 1);
 
+	/*
+	 * file_o is a part of final image we are annotating.
+	 * file_p partially may match that image.
+	 */
 	memset(split, 0, sizeof(struct blame_entry [3]));
 	plno = tlno = 0;
 	for (i = 0; i < patch->num; i++) {
 		struct chunk *chunk = &patch->chunks[i];
 
-		/* tlno to chunk->same are the same as ent */
-		if (ent->num_lines <= tlno)
-			break;
-		if (tlno < chunk->same) {
-			struct blame_entry this[3];
-			split_overlap(this, ent,
-				      tlno + ent->s_lno, plno,
-				      chunk->same + ent->s_lno,
-				      parent);
-			copy_split_if_better(sb, split, this);
-			decref_split(this);
-		}
+		handle_split(sb, ent, tlno, plno, chunk->same, parent, split);
 		plno = chunk->p_next;
 		tlno = chunk->t_next;
 	}
+	/* remainder, if any, all match the preimage */
+	handle_split(sb, ent, tlno, plno, ent->num_lines, parent, split);
 	free_patch(patch);
 }