I used this procedure :

git clone https://github.com/codemirror/CodeMirror.git --branch 5.13.4
cd CodeMirror

# rename edit to cm_edit # see 54087b03
find . -not -path "./.git*" -type f -exec sed -i 's/addon\/edit\//addon\/cm_edit\//g' {} +
git mv addon/edit addon/cm_edit
git commit -a -m "ERP5: rename addon/edit -> addon/cm_edit"

# upload to zope ( note that I had to mkdir addon explicitly, otherwise I had some 409 Conflicts and 500 Internal Server Error that I did not investigate)
lftp http://login:password@172.17.0.2:2200/erp5/portal_skins/erp5_code_mirror/ -e 'rm -rf codemirror ; mkdir codemirror ; quit' &&\
lftp http://login:password@172.17.0.2:2200/erp5/portal_skins/erp5_code_mirror/codemirror/ -e 'mkdir addon; mput -d * */* */*/* */*/*/* ; quit '

# run portal_alarms/check_skin_cache to set http_cache