cli: don't lose short runs of context lines in diffs

If the context was 3 lines or less, it would get lost before this
patch.

src/commands.rs

@@ -697,7 +697,7 @@ fn print_diff(left: &[u8], right: &[u8], styler: &mut dyn Styler) {
                 }
             }
         } else {
-            if context_before {
+            if context_before || context.len() < num_context_lines {
                 for line in &context {
                     print_diff_line(styler, line);
                 }