range-diff --dual-color: work around bogus white-space warning

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.

Work around this by detecting that situation and simply *not* printing
the space in that case.

This is slightly improper a fix because it is conceivable that an
output_prefix might be configured with *just* the right length to let
that tab jump to a different tab stop depending whether we emit that
space or not.

However, the proper fix would be relatively ugly and intrusive because
it would have to *weaken* the WS_SPACE_BEFORE_TAB option in ws.c.
Besides, we do not expose the --dual-color option in cases other than
the `git range-diff` command, which only uses a hard-coded
output_prefix of four spaces (which misses the problem by one
column... ;-)).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

diff.c

@@ -1101,6 +1101,12 @@ 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);
+			/* Avoid space-before-tab warning */
+			if (c == ' ' && (len < 2 || line[1] == '\t' ||
+					 line[1] == '\r' || line[1] == '\n')) {
+				line++;
+				len--;
+			}
 		}
 		emit_line_ws_markup(o, set, reset, line, len, set_sign, '+',
 				    flags & DIFF_SYMBOL_CONTENT_WS_MASK,