merge-tools builtin: omit changes of binary files if hash stayed the same

When the contents of a binary file have not changed (but it appears in
the tree diff because its file mode has) the builtin diff tool still
presented a `Section::Binary` section to the user, showing identical
descriptions for both sides of the diff.

Selecting this section should not affect the selected contents, so it
does not make much sense.

This behavior was not captured by any of the existing snapshot tests but
it will be captured by a new regression test added in the next commit.

Author: jgreitemann

cli/src/merge_tools/builtin.rs

@@ -341,6 +341,18 @@ async fn make_diff_files(
                 sections.extend(make_diff_sections(&old_contents, &new_contents)?);
             }
 
+            (
+                FileContents::Binary {
+                    hash: Some(left_hash),
+                    ..
+                },
+                FileContents::Binary {
+                    hash: Some(right_hash),
+                    ..
+                },
+            ) if left_hash == right_hash => {
+                // Binary file contents have not changed.
+            }
             (left, right @ FileContents::Binary { .. })
             | (left @ FileContents::Binary { .. }, right) => {
                 sections.push(scm_record::Section::Binary {