range-diff: relieve callers of low-level configuration burden

There are a number of very low-level configuration details which need to
be managed precisely to generate a proper range-diff. In particular,
'diff_options' output format, header suppression, indentation, and
dual-color mode must all be set appropriately to ensure proper behavior.

Handle these details locally in the libified range-diff back-end rather
than forcing each caller to have specialized knowledge of these
implementation details, and to avoid duplication as new callers are
added.

While at it, localize these tweaks to be active only while generating
the range-diff, so they don't clobber the caller-provided
'diff_options', which might be used beyond range-diff generation.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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