Magit misbehavior on MacOS - newbie here

[Kevin Buchs <kevin.buchs@gmail.com>]

I just started using magit a few days ago and it is working fine on Linux
(emacs v26). However on my Mac (emacs v25.3.1, magit 2.13.0) it is
misbehaving when I try to perform a commit. It seems to be taking a script
and instead opening buffers with the words found in the script, as well as
trying to create files by the same name. The literal buffer/file names
include:

$sleep'
wait
USR2;
1"
exit
$sleep;
"kill
trap
USR1;
0"
sleep=$!;
&
604800
sleep
$0";
OPEN
$$
"WITH-EDITOR:
'echo

These seem to be coming from
~/lib/emacs/elpa/with-editor-2.7.3/with-editor.el, which includes this
defcustom:

(defcustom with-editor-sleeping-editor "\
sh -c '\
echo \"WITH-EDITOR: $$ OPEN $0\"; \
sleep 604800 & sleep=$!; \
trap \"kill $sleep; exit 0\" USR1; \
trap \"kill $sleep; exit 1\" USR2; \
wait $sleep'"

and ~/lib/emacs is the first element of my load-path.

Anyone have ideas on this problem?

Kevin Buchs