[snip]

+	unsigned mbase_null = !(mask & 1);
+	unsigned side1_null = !(mask & 2);
+	unsigned side2_null = !(mask & 4);

Should these be "int"?

+	/*
+	 * A bunch of sanity checks verifying that traverse_trees() calls
+	 * us the way I expect.  Could just remove these at some point,
+	 * though maybe they are helpful to future code readers.
+	 */
+	assert(mbase_null == is_null_oid(&names[0].oid));
+	assert(side1_null == is_null_oid(&names[1].oid));
+	assert(side2_null == is_null_oid(&names[2].oid));
+	assert(!mbase_null || !side1_null || !side2_null);
+	assert(mask > 0 && mask < 8);

These were helpful to me.

+	/* Other invariant checks, mostly for documentation purposes. */
+	assert(mask == (dirmask | filemask));

But not this - filemask was computed in this function, so I need not
look elsewhere to see that this is correct.

+	/*
+	 * TODO: record information about the path other than all zeros,
+	 * so we can resolve later in process_entries.
+	 */
+	ci = xcalloc(1, sizeof(struct conflict_info));
+	strmap_put(&opti->paths, fullpath, ci);

OK - so each entry is a full-size conflict_info to store all relevant
information. Presumably some of these will be converted later into what
is effectively a struct merged_info (so, the extra struct conflict_info
fields are unused but memory is still occupied).

I do see that in patch 10, there is an optimization that directly
allocates the smaller struct merged_info when it is known at this point
that there is no conflict.

[snip rest of function]

 static int collect_merge_info(struct merge_options *opt,
 			      struct tree *merge_base,
 			      struct tree *side1,
 			      struct tree *side2)
 {
-	/* TODO: Implement this using traverse_trees() */
-	die("Not yet implemented.");
+	int ret;
+	struct tree_desc t[3];
+	struct traverse_info info;
+	char *toplevel_dir_placeholder = "";
+
+	opt->priv->current_dir_name = toplevel_dir_placeholder;
+	setup_traverse_info(&info, toplevel_dir_placeholder);

I thought that this was written like this (instead of inlining the 2
double-quotes) to ensure that the string-equality-is-pointer-equality
characteristic holds, but I see that that characteristic is for
directory_name in struct merged_info, not current_dir_name in struct
merge_options_internal. Any reason for not inlining ""?

[snip rest of function]