How often does git calculate SHAs?

How often does git actually calculate a file's SHA?

`strace git status` shows that git stat'ed many files, but opened only a few. So I assume git has some heuristicsbased on the stat infofor when to recalculate the SHAs.

Any pointers to how I could have figured this out myself from looking at the source code would be appreciated. Google wasn't helpful.

Michael