Fix wrap symbol background on homolog pairs in side-by-side

[igrmk]

Wrap symbols in side-by-side in homolog should inherit the line's non-emph background

Symptom

In side-by-side mode only, in a homolog, the wrap continuation symbol is painted as minus-style / plus-style
while the surrounding content is in minus-non-emph-style / plus-non-emph-style.

The bug is only visible when minus-style and minus-non-emph-style (or plus-style and plus-non-emph-style)
are configured with different background colors.
Delta's defaults give each pair the same background — so the bug appears only in custom themes.

Self-contained reproduction against this repo at terminal width 160:

COLUMNS=160 git --no-pager show d2765a48 -- src/cli.rs | head -30 \
  | delta --no-gitconfig --side-by-side --width 160 \
      --minus-style          'white "#785c5a"' \
      --minus-emph-style     'bold white "#785c5a"' \
      --minus-non-emph-style 'white "#454550"' \
      --plus-style           'white "#5b6d5a"' \
      --plus-emph-style      'bold white "#5b6d5a"' \
      --plus-non-emph-style  'white "#454550"'

Before

After

Root cause

wrap_minusplus_block (src/wrapping.rs) hard-codes fill_style to {minus,plus}_style regardless of whether the pair has a homolog.
That fill_style becomes the background of the wrap continuation symbol and any right-aligned padding spaces.

But when a minus/plus pair has a homolog, the rest of the painted bg is {minus,plus}_non_emph_style
(set by Painter::get_should_right_fill_background_color_and_fill_style in paint.rs via the line_has_homolog == Some(true) branch).
So the wrap symbols and padding pick a bg that doesn't match the line they're embedded in.

Fix

Compute fill_style per alignment pair:

• (Some, None) or (None, Some) — unpaired line, no homolog: keep {minus,plus}_style.
• (Some, Some) — paired with homolog: use {minus,plus}_non_emph_style.