Fix crash when diff touches first line

CHANGELOG.md

@@ -3,6 +3,8 @@
 Fixed an issue where complex diffs would not display some unchanged
 lines.
 
+Fixed a crash when diff context included the first line.
+
 ## 0.3
 
 Diffs are now displayed with unchanged lines aligned to the other side.

src/syntax.rs

@@ -556,16 +556,16 @@ pub fn aligned_lines(
     rhs_lines: &[LineNumber],
     lhs_line_matches: &HashMap<LineNumber, LineNumber>,
 ) -> Vec<(Option<LineNumber>, Option<LineNumber>)> {
-    let mut rhs_highest_matched = rhs_lines.first().unwrap().number - 1;
+    let mut rhs_highest_matched = rhs_lines.first().unwrap().number as isize - 1;
     // For every LHS line, if there is a RHS line that is included in
     // `rhs_lines` and hasn't yet been paired up, add it to
     // matched_lines.
     let mut matched_lines = vec![];
     for lhs_line in lhs_lines {
         if let Some(rhs_line) = lhs_line_matches.get(lhs_line) {
-            if rhs_line.number > rhs_highest_matched {
+            if rhs_line.number as isize > rhs_highest_matched {
                 matched_lines.push((lhs_line, rhs_line));
-                rhs_highest_matched = rhs_line.number;
+                rhs_highest_matched = rhs_line.number as isize;
             }
         }
     }
@@ -770,6 +770,20 @@ mod tests {
         );
     }
 
+    #[test]
+    fn test_aligned_first_line() {
+        let lhs_lines: Vec<LineNumber> = vec![0.into()];
+        let rhs_lines: Vec<LineNumber> = vec![0.into()];
+
+        let mut line_matches: HashMap<LineNumber, LineNumber> = HashMap::new();
+        line_matches.insert(0.into(), 0.into());
+
+        assert_eq!(
+            aligned_lines(&lhs_lines, &rhs_lines, &line_matches),
+            vec![(Some(0.into()), Some(0.into()))]
+        );
+    }
+
     /// Ensure that we assign prev_opposite_pos even if the change is on the first node.
     #[test]
     fn test_prev_opposite_pos_first_node() {