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

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Michael Cook, Raytheon BBN Technologies, www.bbn.com