Wrong text encoding with ly:gulp-file

%% Greetings everybody, this one’s rather funny:

\version "2.25.25"

Ill = "I’ll"

\markup { \Ill be }

#(ly:parser-include-string "Ill = \"I’ll\"")

\markup { \Ill be }

incFile = "/tmp/incl.txt"

#(let ((output-port (open-file incFile "a")))
    (display "Ill = \"I’ll\"" output-port)
    (newline output-port)
    (close output-port))

#(ly:parser-include-string (ly:gulp-file incFile))

\markup { \Ill be }

%% Well, I’ll be…

Cheers,
-- V. V.