log -L: improve comments in process_all_files()

The funny range assignment in process_all_files() had me sidetracked
while investigating what led to the previous commit.  Let's improve
the comments.

Signed-off-by: Thomas Rast <trast@inf.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-log.c

@@ -1095,11 +1095,24 @@ static int process_all_files(struct line_log_data **range_out,
 
 	for (i = 0; i < queue->nr; i++) {
 		struct diff_ranges *pairdiff = NULL;
-		if (process_diff_filepair(rev, queue->queue[i], *range_out, &pairdiff)) {
+		struct diff_filepair *pair = queue->queue[i];
+		if (process_diff_filepair(rev, pair, *range_out, &pairdiff)) {
+			/*
+			 * Store away the diff for later output.  We
+			 * tuck it in the ranges we got as _input_,
+			 * since that's the commit that caused the
+			 * diff.
+			 *
+			 * NEEDSWORK not enough when we get around to
+			 * doing something interesting with merges;
+			 * currently each invocation on a merge parent
+			 * trashes the previous one's diff.
+			 *
+			 * NEEDSWORK tramples over data structures not owned here
+			 */
 			struct line_log_data *rg = range;
 			changed++;
-			/* NEEDSWORK tramples over data structures not owned here */
-			while (rg && strcmp(rg->path, queue->queue[i]->two->path))
+			while (rg && strcmp(rg->path, pair->two->path))
 				rg = rg->next;
 			assert(rg);
 			rg->pair = diff_filepair_dup(queue->queue[i]);