Make bad position data more obvious (use purple)

CHANGELOG.md

@@ -10,6 +10,12 @@ Added support for Mercuial, see [this section in the
 manual](http://difftastic.wilfred.me.uk/getting_started.html#mercurial-external-diffs)
 for instructions.
 
+### Display
+
+Characters that don't have a position in the parsed syntax tree are
+now displayed in purple, to make bugs more obvious. Previously they
+were dimmed.
+
 ## 0.10.1
 
 ### Build

src/style.rs

@@ -32,7 +32,7 @@ impl Style {
 /// Dim any parts of the line that have no spans.
 fn apply_line(line: &str, styles: &[(SingleLineSpan, Style)]) -> String {
     if styles.is_empty() {
-        return line.dimmed().to_string();
+        return line.purple().to_string();
     }
 
     let mut res = String::with_capacity(line.len());