Either mu[m's] git or magi{c => t} is fine.
The slogan is "It’s Magit! The magical Git client", so it makes sense to pronounce Magit like magic, while taking into account that C and T do not sound the same.
The German "Magie" is not pronounced the same as the English "magic",
so if you speak German then you can use the above rationale to justify
using the former pronunciation; Mag{ie => it}.
You can also choose to use the former pronunciation just because you like it better.
Also see https://magit.vc/assets/videos/magic.mp4. Also see https://emacs.stackexchange.com/questions/13696.