rerere: explain "rerere forget" codepath

Explain the internals of rerere as in-code comments, while
sprinkling "NEEDSWORK" comment to highlight iffy bits and
questionable assumptions.

This covers the codepath that implements "rerere forget".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

rerere.c

@@ -422,6 +422,10 @@ static int handle_cache(const char *path, unsigned char *sha1, const char *outpu
 	strbuf_init(&io.input, 0);
 	strbuf_attach(&io.input, result.ptr, result.size, result.size);
 
+	/*
+	 * Grab the conflict ID and optionally write the original
+	 * contents with conflict markers out.
+	 */
 	hunk_no = handle_path(sha1, (struct rerere_io *)&io, marker_size);
 	strbuf_release(&io.input);
 	if (io.io.output)
@@ -786,19 +790,34 @@ static int rerere_forget_one_path(const char *path, struct string_list *rr)
 	int ret;
 	struct string_list_item *item;
 
+	/*
+	 * Recreate the original conflict from the stages in the
+	 * index and compute the conflict ID
+	 */
 	ret = handle_cache(path, sha1, NULL);
 	if (ret < 1)
 		return error("Could not parse conflict hunks in '%s'", path);
+
+	/* Nuke the recorded resolution for the conflict */
 	hex = xstrdup(sha1_to_hex(sha1));
 	filename = rerere_path(hex, "postimage");
 	if (unlink(filename))
 		return (errno == ENOENT
 			? error("no remembered resolution for %s", path)
 			: error("cannot unlink %s: %s", filename, strerror(errno)));
 
+	/*
+	 * Update the preimage so that the user can resolve the
+	 * conflict in the working tree, run us again to record
+	 * the postimage.
+	 */
 	handle_cache(path, sha1, rerere_path(hex, "preimage"));
 	fprintf(stderr, "Updated preimage for '%s'\n", path);
 
+	/*
+	 * And remember that we can record resolution for this
+	 * conflict when the user is done.
+	 */
 	item = string_list_insert(rr, path);
 	free(item->util);
 	item->util = hex;
@@ -817,6 +836,11 @@ int rerere_forget(struct pathspec *pathspec)
 
 	fd = setup_rerere(&merge_rr, RERERE_NOAUTOUPDATE);
 
+	/*
+	 * The paths may have been resolved (incorrectly);
+	 * recover the original conflicted state and then
+	 * find the conflicted paths.
+	 */
 	unmerge_cache(pathspec);
 	find_conflict(&conflict);
 	for (i = 0; i < conflict.nr; i++) {