Use proper temp names for diff temp files

The previous patches (almost 20 years ago) intending to do this didn't
quite finish the job. The added bonus with proper temp names is that
they will be included in the automatic cleanup, should they be left
behind for some reason.

src/files.ml

@@ -841,8 +841,8 @@ let rec diff root1 path1 ui1 root2 path2 ui2 showDiff id =
         (fun () ->
            let path1 = Update.translatePathLocal fspath1 path1 in
            let (workingDir, realPath) = Fspath.findWorkingDir fspath1 path1 in
-           let tmppath =
-             Path.addSuffixToFinalName realPath (tempName "diff-") in
+           let tmppath = Os.tempPath ~fresh:false workingDir
+             (Path.addSuffixToFinalName realPath "-diff") in
            Os.delete workingDir tmppath;
            Lwt_unix.run
              (Update.translatePath root2 path2 >>= (fun path2 ->
@@ -860,8 +860,8 @@ let rec diff root1 path1 ui1 root2 path2 ui2 showDiff id =
         (fun () ->
            let path2 = Update.translatePathLocal fspath2 path2 in
            let (workingDir, realPath) = Fspath.findWorkingDir fspath2 path2 in
-           let tmppath =
-             Path.addSuffixToFinalName realPath "#unisondiff-" in
+           let tmppath = Os.tempPath ~fresh:false workingDir
+             (Path.addSuffixToFinalName realPath "-diff") in
            Lwt_unix.run
              (Update.translatePath root1 path1 >>= (fun path1 ->
               (* Note that we don't need the resource fork *)