range-diff --dual-color: skip white-space warnings

range-diff --dual-color: skip white-space warnings

When displaying a diff of diffs, it is possible that there is an outer
`+` before a context line. That happens when the context changed between
old and new commit. When that context line starts with a tab (after the
space that marks it as context line), our diff machinery spits out a
white-space error (space before tab), but in this case, that is
incorrect.

Rather than adding a specific whitespace flag that specifically ignores
the first space in the output (and might miss other problems with the
white-space warnings), let's just skip handling white-space errors in
dual color mode to begin with.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>