Have to consider this:
http://forum.freegamedev.net/viewtopic.php?f=38&p=10193#p10177 too:
Until this point I tried to keep game data small and put it into the same (git) repository.
So far I have just separated the "data"-folder further into a "base" directory and "packages".
"base" contains critical small footprint data, without which the game would not "boot" and "testrun" at all - this should be considered part of the code.
"packages" are in the form of "opengameart.org"-, "freesound.org"-, and "blendswap.com"-directories and contain contributed data/art with large memory footprint without which the game could still run (but without all funkiness).
So separating package-data from source-code + base-data is a point I should consider for version-control.
This takes me to another possible topic and requirement:
The game shall be able to replace crude base-data with refined package-data when available.
Maybe just overwrite base-data at package-installation or decide at runtime.
Eg. Foliage textures in the base-directory are 64x64 pixels large, and there may be a hi-res replacement file in a package.