.gitconfig: Use `diff-highlight` contrib script as diff-filter

.gitconfig: Use `diff-highlight` contrib script as diff-filter

- bin/diff-filter: Wrapper around calling the Git `diff-highlight`
  contrib script. This will degrade gracefully if `diff-highlight`
  script cannot be found in standard locations.

- bin/diff-pager: Wrapper around calling new `diff-filter | $PAGER`.
  This can be used via `git config core.pager` or any other
  pager-like places which want to format git-patch output.

- .gitconfig:
  - core.pager: use new `diff-pager` wrapper script.
  - interactive.diffFilter: use new `diff-filter` wrapper script.
    This lets `git add -i ...` use the same shared diff-filter.

  - color.diff-highlight: Tweak colors so that "normal" colors have
    a normal [non-colored] foreground color, and "highlight" colors
    can be simply "red bold" and "green bold" respectively. This
    eliminates visual clutter from the colored diff display, so that
    the diff output is less noise [for line-pairs which
    `diff-highlight` can process].