Preserve comments when rewriting opam files

Signed-off-by: Jon Ludlam <jon@recoil.org>

dune.opam

@@ -11,6 +11,7 @@ depends: [
   "base-threads"
 ]
 build: [
+  # opam 2 sets OPAM_SWITCH_PREFIX, so we don't need a hardcoded path
   ["ocaml" "configure.ml" "--libdir" lib] {opam-version < "2"}
   ["ocaml" "bootstrap.ml"]
   ["./boot.exe" "--release" "--subst"] {pinned}
@@ -20,6 +21,7 @@ conflicts: [
   "jbuilder" {!= "transition"}
   "odoc" {< "1.3.0"}
 ]
+
 synopsis: "Fast, portable and opinionated build system"
 description: """
 dune is a build system that was designed to simplify the release of
@@ -38,4 +40,4 @@ It also supports multi-context builds, such as building against
 several opam roots/switches simultaneously. This helps maintaining
 packages across several versions of OCaml and gives cross-compilation
 for free.
-"""
+"""

src/opam_create.ml

@@ -32,7 +32,7 @@ let add_rules sctx ~dir ~project =
         let package_name = Local_package.name pkg in
         let corrected =
           Opam_file.Mutator.apply (correct project package_name) opamfile in
-        OpamPrinter.opamfile corrected) >>>
+        OpamPrinter.Preserved.items contents opamfile.file_contents corrected.file_contents) >>>
       Build.write_file_dyn expected_path
     in
     let diff_rule =