Tamriel Data
Tamriel Data is the shared repository used by Project Tamriel, Tamriel Rebuilt, and third party mods. It provides an additional base of models and items to work from, in addition to everything already provided by Morrowind, Tribunal, and Bloodmoon.
TD_Addon
The TD_Addon git repository is continuously updated as more assets are merged. Developers are encouraged to stay on top of these changes to prevent missing meshes, textures, objects, and such, and to stay up-to-date with deprecations.
Downloading the addon
While it is possible to download the addon straight from the browser using the green Code button, developers are encouraged to use git so only changes have to be downloaded instead of every file every time. The easiest way to do this is to use GitHub Desktop which provides a GUI that lets you "Pull" in changes. Alternatively, the downloads listed on the Git website might also be used.
Automatic redating
Git does not store file dates, meaning that every change to Tamriel_Data.esm changes the modification date to the present. Given that Morrowind.exe determines your load order based on this date, that can be a headache. It's possible to create a hook that runs a script that redates the ESM every time you pull in changes. To do so, create a file named post-merge (no extension) in TD_Addon\.git\hooks. Adding the following script to that file will redate the ESM to 2012-01-01 17:30 on every pull. Changing this to a date/time more suitable to your load order is left as an exercise to the reader.
#!/bin/sh touch -m -t 201201011730 "00 Data Files/Tamriel_Data.esm"