git-cvsserver: port to SHA-256

git-cvsserver: port to SHA-256

The code of git-cvsserver currently has several hard-coded 20 and 40
constants that are the length of SHA-1.  When parsing the configuration
file, read the extensions.objectformat configuration setting as well as
CVS-related ones and adjust the hash sizes accordingly.  Use these
computed values in all the places we match object IDs.

Signed-off-by: brian m. carlson <redacted>
Acked-by: Eric Wong <redacted>
Signed-off-by: Junio C Hamano <redacted>