Be defensive against extremely narrow (<10) terminal widths

Previously this would overflow.

src/display/side_by_side.rs

@@ -203,9 +203,23 @@ impl SourceDimensions {
         let rhs_line_nums_width = format_line_num(rhs_max_line).len();
 
         let lhs_total_width = (terminal_width - SPACER.len()) / 2;
-        let lhs_content_width = lhs_total_width - lhs_line_nums_width;
-        let rhs_content_width =
-            terminal_width - lhs_total_width - SPACER.len() - rhs_line_nums_width;
+
+        let lhs_content_width = if lhs_line_nums_width < lhs_total_width {
+            lhs_total_width - lhs_line_nums_width
+        } else {
+            // The terminal is so narrow that even the column numbers
+            // display doesn't fit. Ensure we show a non-zero number
+            // of content columns anyway.
+            1
+        };
+
+        let rhs_content_width = max(
+            1,
+            terminal_width as isize
+                - lhs_total_width as isize
+                - SPACER.len() as isize
+                - rhs_line_nums_width as isize,
+        ) as usize;
 
         Self {
             lhs_content_width,