🐛 Don't apply underline and strike-through to leading whitespace

I think it would be nice not to apply the underline to the leading whitespace here:

Start by reproducing the underline and strike-through behavior described in issue #787 using the linked image as the expected reference. Trace the Rust code responsible for rendering styled diff output; done means leading whitespace remains unstyled while the intended text keeps its underline or strike-through.

Written by the indexing model from the issue text.