Termux is an Android app that can run some Linux software in a terminal.
git-annex is not currently part of the Termux distribution, but it's easy to install it. Paste these commands into Termux:
pkg install wget
wget https://git-annex.branchable.com/install/Android/git-annex-install.sh
sh git-annex-install.sh
Now you can set up a git-annex repository or two, to manage your Android files. To avoid typing any more, you can start up the git-annex webapp and set things up in there:
git annex webapp
Or you can continue using git-annex at the command line. Here we'll make a repository for photos:
cd ~/storage/dcim
git init
git annex init
You can go on to set up a ssh remote pointing to a server, and sync your files to and from it.
And so on. Most ways you would use git-annex on a Linux system work fairly well in this Termux environment.
Starting on boot
If you install the Termux:Boot app, git-annex will also integrate with it. Repositories you create with the webapp (or listed in ~/.config/git-annex/autostart) will have the assistant started in them on boot, so you don't need to start it manually.
Known limitations
Direct mode
git-annex will probably default to direct mode due to the limitations of Android's /sdcard. You can get around this using v6 unlocked repositories, or by putting the git-annex repository on a better filesystem on the Android device, such as your termux home directory.
Upgrading
To upgrade to a new git-annex release, just run
git-annex-install.sh
again.
Bunch of old comments about problems with this that have been fixed were moved to old comments.