Making literal "20" symbolic

Hello,

I notice that there are a lot of uses of the literal 20 throughout git; I'd like to change them (as appropriate) to HASH_WIDTH, or similar; and maybe HASH_WIDTH_ASCII for the 40s.

Is there a particular header file that is appropriate to put 

#define HASH_WIDTH 20 #define HASH_WIDTH_ASCII (HASH_WIDTH*2)

Of course, I plan to review each instance to make sure I'm not changing a non-hash width 20.

Andy