Always preserve expanded inherits

This changes the formatting to take into account whether inherits
are expanded onto multiple lines already, and if so, preserves that.

This is desirable because if a user writes an inherit over
multiple lines, there's most likely an intention to add more,
which would become harder when formatted.

An example of this is inheriting from `lib`, which usually starts out
with just a single value, but usually gets more later. This would've been
contracted onto a single line before this change:

    inherit (lib)
      foo
      ;

This change conforms to the standard due to this line:

> The formatter may take the input formatting into account in some cases in order to preserve multi-line syntax elements (which would otherwise have been contracted by the rules).

src/Nixfmt/Pretty.hs

@@ -129,8 +129,11 @@ instance Pretty Binder where
       pretty inherit
         <> ( if null ids
               then pretty semicolon
-              else line <> nest (sepBy (if length ids < 4 then line else hardline) ids <> line' <> pretty semicolon)
+              else sep <> nest (sepBy sep ids <> nosep <> pretty semicolon)
            )
+    where
+      -- Only allow a single line if it's already on a single line and has few enough elements
+      (sep, nosep) = if sourceLine inherit == sourceLine semicolon && length ids < 4 then (line, line') else (hardline, hardline)
   -- `inherit (foo) bar` statement
   pretty (Inherit inherit (Just source) ids semicolon) =
     group $
@@ -140,11 +143,14 @@ instance Pretty Binder where
               <> if null ids
                 then pretty semicolon
                 else
-                  line
-                    <> sepBy (if length ids < 4 then line else hardline) ids
-                    <> line'
+                  sep
+                    <> sepBy sep ids
+                    <> nosep
                     <> pretty semicolon
           )
+    where
+      -- Only allow a single line if it's already on a single line and has few enough elements
+      (sep, nosep) = if sourceLine inherit == sourceLine semicolon && length ids < 4 then (line, line') else (hardline, hardline)
   -- `foo = bar`
   pretty (Assignment selectors assign expr semicolon) =
     group $

test/diff/inherit/in.nix

@@ -29,4 +29,14 @@
       h
       ;
   }
+
+  {
+    inherit
+      ;
+    inherit
+      a;
+
+    inherit a
+      ;
+  }
 ]

test/diff/inherit/out.nix

@@ -19,7 +19,9 @@
       ;
   }
   {
-    inherit aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa;
+    inherit
+      aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
+      ;
   }
   { inherit b d; }
   {
@@ -79,4 +81,15 @@
       h
       ;
   }
+
+  {
+    inherit;
+    inherit
+      a
+      ;
+
+    inherit
+      a
+      ;
+  }
 ]