Here is the way I found to successfully update MobileFrontend via git.
cd extensions rm -r MobileFrontend git clone --depth 1 https://gerrit.wikimedia.org/r/p/mediawiki/extensions/MobileFrontend.git
That is right, reinstalling from scratch.
I originally thought just git pull would update it. But it does not.
If there is a better way it perhaps could be mentioned somewhere in https://www.mediawiki.org/wiki/Extension:MobileFrontend#Download https://www.mediawiki.org/wiki/Manual:Upgrading#Upgrade_extensions , https://www.mediawiki.org/wiki/Download_from_Git#Keeping_up_to_date . https://www.mediawiki.org/wiki/Download_from_Git#Using_Git_to_download_Media... mentions git submodule update --init --recursive but that did not help.
Anyway, no big deal. reinstalling isn't to net-intensive.