diff: print context lines also when there are exactly 3 lines

My recent fix to print context lines when there are less than 3 lines
of context wasn't enough; we should also print context lines when
there are exactly 3 lines of context :) I can't understand what the
`!context_before` condition was for, so I've just removed it. I guess
I'll notice soon if things look worse in some case.

src/commands.rs

@@ -710,11 +710,9 @@ fn print_diff(left: &[u8], right: &[u8], styler: &mut dyn Styler) -> io::Result<
                 }
             }
         } else {
-            if context_before || context.len() < num_context_lines {
             for line in &context {
                 print_diff_line(styler, line)?;
             }
-            }
             context.clear();
             print_diff_line(styler, &diff_line)?;
             context_before = false;