Remove an assert false, and silence warning 8

src/git_kv.ml

@@ -472,8 +472,10 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
            ~none:Ptime.epoch
            ~some:Fun.id (Ptime.of_float_s (Int64.to_float (now ()))))
     | None, Some head ->
-      Store.read_exn t.store head >|= function
-      | Commit c ->
+      (* See https://github.com/ocaml/ocaml/issues/9301 why we have the
+         intermediate [r] value. *)
+      let+ r = Store.read_exn t.store head in
+      let[@warning "-8"] Commit c = r in
       let author = Git_commit.author c in
       let secs, tz_offset = author.Git.User.date in
       let secs =
@@ -491,7 +493,6 @@ module Make (Pclock : Mirage_clock.PCLOCK) = struct
           ~some:Fun.id (Ptime.of_float_s (Int64.to_float secs))
       in
       Ok ts
-      | _ -> assert false
 
   let digest t key =
     let open Lwt.Infix in