This app uses this fork of the libgit2 library. You can compile it from source or use the binaries already in place in the jniLibs folder.
I all case, you will need to clone this repo to get the header.
git clone https://github.com/wiiznokes/libgit2-android --depth=1 --branch=patch-android
You can't just sudo apt install libgit2-dev because cmake won't let you use the header in /usr/include for some reason.
I you want to compile libgit2, good luck, you can find info in the build.sh script.