Make 1-indexed display of LineNumber values explicit

This fixes .. (or 'missing') line numbers on boundaries, where we'd
show too few dots after 10 or 100.

src/lines.rs

@@ -10,6 +10,12 @@ use std::{cmp::max, fmt};
 #[derive(Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash)]
 pub struct LineNumber(pub usize);
 
+impl LineNumber {
+    pub fn one_indexed(&self) -> usize {
+        self.0 + 1
+    }
+}
+
 impl fmt::Debug for LineNumber {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.write_fmt(format_args!("LineNumber: {}", self.0))
@@ -23,7 +29,7 @@ impl From<usize> for LineNumber {
 }
 
 pub fn format_line_num(line_num: LineNumber) -> String {
-    format!("{} ", line_num.0 + 1)
+    format!("{} ", line_num.one_indexed())
 }
 
 /// A position in a single line of a string.

src/side_by_side.rs

@@ -37,11 +37,15 @@ fn split_lines_nonempty(s: &str) -> Vec<String> {
 }
 
 fn format_line_num_padded(line_num: LineNumber, column_width: usize) -> String {
-    format!("{:width$} ", line_num.0 + 1, width = column_width - 1)
+    format!(
+        "{:width$} ",
+        line_num.one_indexed(),
+        width = column_width - 1
+    )
 }
 
 fn format_missing_line_num(prev_num: LineNumber, column_width: usize) -> String {
-    let num_digits = format!("{}", prev_num.0).len();
+    let num_digits = format!("{}", prev_num.one_indexed()).len();
     format!(
         "{:>width$} ",
         ".".repeat(num_digits),