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

diff.c

@@ -1299,6 +1299,7 @@ static void emit_diff_symbol_from_struct(struct diff_options *o,
 				set = diff_get_color_opt(o, DIFF_FRAGINFO);
 			else if (c != '+')
 				set = diff_get_color_opt(o, DIFF_CONTEXT);
+			flags &= ~DIFF_SYMBOL_CONTENT_WS_MASK;
 		}
 		emit_line_ws_markup(o, set, reset, line, len, set_sign, '+',
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK,