[PATCH] sanitize --abbrev option for describe

Hi,

the --abbrev option doesn't allow a 40 digit SHA1 output.

The following patch fixes this, and uses a IMHO more logical way to 
compute the abbrev parameter, so --abbrev=52 will still yield a 40 digit 
SHA1.


-- 
  Han-Wen Nienhuys - hanwen@xs4all.nl - http://www.xs4all.nl/~hanwen