Use tactic instead of config

Add mixed test
Mixed is unreachable as there is no input combination that could get
to this value

src/expr.rs

@@ -1335,9 +1335,10 @@ fn rewrite_struct_lit<'a>(context: &RewriteContext,
         },
         indent: indent,
         width: budget,
-        ends_with_newline: match context.config.struct_lit_style {
-            StructLitStyle::Block => true,
-            StructLitStyle::Visual => false,
+        ends_with_newline: match tactic {
+            DefinitiveListTactic::Horizontal => false,
+            DefinitiveListTactic::Vertical => true,
+            DefinitiveListTactic::Mixed => unreachable!(),
         },
         config: context.config,
     };

tests/source/struct_lits.rs

@@ -110,4 +110,7 @@ fn issue491() {
     Foo {
         arm: 0, // Comment
     };
+
+    Foo { a: aaaaaaaaaa, b: bbbbbbbb, c: cccccccccc, d: dddddddddd, /* a comment */
+      e: eeeeeeeee };
 }

tests/target/struct_lits.rs

@@ -140,4 +140,12 @@ fn issue491() {
     };
 
     Foo { arm: 0 /* Comment */ };
+
+    Foo {
+        a: aaaaaaaaaa,
+        b: bbbbbbbb,
+        c: cccccccccc,
+        d: dddddddddd, // a comment
+        e: eeeeeeeee,
+    };
 }