[subst] [test] Add test for #3219

Signed-off-by: Emilio Jesus Gallego Arias <e+git@x80.org>

test/blackbox-tests/test-cases/subst.t/run.t

@@ -70,3 +70,40 @@ And without an opam file preset.
   (package (name foo) (authors "John Doe <john@doe.com>"))
 
   $ rm -rf .git
+
+Test subst and files with unicode (#3879)
+-----------------------------------------
+
+  $ rm -f foo.opam
+
+  $ cat > dune-project <<EOF
+  > (lang dune 2.0)
+  > (name foo)
+  > (package (name foo) (authors "John Doe <john@doe.com>"))
+  > EOF
+
+  $ X=%%; cat > α-term.ml <<EOF
+  > let name = "${X}NAME${X}"
+  > let authors = "${X}PKG_AUTHORS${X}"
+  > let version = "${X}VERSION${X}"
+  > EOF
+
+  $ git init --quiet
+  $ git add .
+  $ git commit -am _ --quiet
+  $ git tag -a 1.0 -m 1.0
+
+  $ dune subst
+
+  $ cat α-term.ml
+  let name = "foo"
+  let authors = "John Doe <john@doe.com>"
+  let version = "1.0"
+
+  $ cat dune-project
+  (lang dune 2.0)
+  (name foo)
+  (version 1.0)
+  (package (name foo) (authors "John Doe <john@doe.com>"))
+
+  $ rm -rf .git