Ensure LHS and RHS source code are the same width

CHANGELOG.md

@@ -6,6 +6,9 @@ Side-by-side display now uses "..." for column numbers when aligning
 lines. This makes hunks more obvious, but hunks now also have two
 blank lines between them to make it clearer.
 
+Fixed an issue where screen width was not shared evenly by LHS and
+RHS.
+
 ### Parsing
 
 Fixed handling of `->` in Rust.

src/side_by_side.rs

@@ -53,7 +53,7 @@ fn lhs_printable_width(
     let longest_line_length = longest_visible_line_lhs(lhs, groups);
     let longest_line = longest_line_length + lhs_column_width;
 
-    let space_available = terminal_width / 2 - SPACER.len();
+    let space_available = (terminal_width - SPACER.len()) / 2;
     max(MIN_WIDTH, min(longest_line, space_available))
 }