Just doing a search for that error it looks like it is probably a problem with your .git/index. You should be able to fix the “bad index” issue by running:

cd ~/MagicMirror rm -rf .git/index git reset

You may want to make a backup of your config.js file just in case it does anything weird. I’m not sure if that will solve your module problem. It should allow you to pull the latest MagicMirror version though.

@jeremy
I got stuck on something like that too…
a) read the thread https://forum.magicmirror.builders/topic/254/sachin-s-mirror/25

b) try a module @KirAsh4 wrote for me (and that I changed in a miniscule way)
https://forum.magicmirror.builders/topic/373/alternating-modules-to-save-on-real-estate

Good luck!