Ensure side-by-side-show-both always uses two columns

Fixes #812

CHANGELOG.md

@@ -4,6 +4,11 @@
 
 When diffing directories, difftastic now ignores the `.git` directory.
 
+### Display
+
+Fixed an issue where `--display=side-by-side-show-both` would not use
+a two-column display when one file was empty.
+
 ### Command Line Interface
 
 Difftastic no longer accepts the `--missing-as-empty`. This has had no

src/display/side_by_side.rs

@@ -388,7 +388,12 @@ pub(crate) fn print(
         .map(|l| replace_tabs(l, display_options.tab_width))
         .collect();
 
-    if lhs_src.is_empty() {
+    if lhs_src.is_empty()
+        && !matches!(
+            display_options.display_mode,
+            DisplayMode::SideBySideShowBoth
+        )
+    {
         for line in display_single_column(
             display_path,
             old_path,
@@ -402,7 +407,12 @@ pub(crate) fn print(
         println!();
         return;
     }
-    if rhs_src.is_empty() {
+    if rhs_src.is_empty()
+        && !matches!(
+            display_options.display_mode,
+            DisplayMode::SideBySideShowBoth
+        )
+    {
         for line in display_single_column(
             display_path,
             old_path,