refactor: avoid strange round-trip of draw_fn arguments

[Integral-Tech]

• Although format! macro is very powerful, it introduces overhead of memory allocation on the heap compared to &str. Therefore, when format! macro is unnecessary, we should avoid using it.

[th1000s]

I noticed that this pattern repeats itself:

let (draw_fn, pad, style) = draw::get_draw_function();

draw_fn(pad, style, ...);

The get returns a function/closure and parameters for it, so those are (directly or indirectly) fed back into it. Instead the returned closure should already handle these, then the if pad { .. } else { .. } is only inside the closure, and decoration_ansi_term_style doesn't make this strange round-trip.

Also, type DrawFunction doesn't have to be a dyn FnMut, just dyn Fn. However, the padding in merge_conflict.rs would also get applied to derived_commit_name, but maybe this visually doesn't make a difference.

[Integral-Tech]

@th1000s I have just submitted a commit to fix the strange round-trip :)