Fix another crash on empty files

Git history with file additions/removals should now work.

CHANGELOG.md

@@ -3,7 +3,7 @@
 ### Parsing
 
 Fixed a crash on parsing non-ASCII source files. Fixed a crash on
-files without an extension.
+files without an extension. Fixed crashes on empty files.
 
 Improved parsing for Rust punctuation.
 

src/syntax.rs

@@ -712,7 +712,8 @@ 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 as isize - 1;
+    let mut rhs_highest_matched = rhs_lines.first().map(|l| l.number as isize).unwrap_or(0) - 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.