use git's default, collision-detecting SHA1 implementation
    
    Now that the bundled git is > 2.13.0 we can drop the BLK_SHA1 make
    variable¹.  The git default is now DC_SHA1 which provides collision
    detection to help protect against the SHATTERED attack.
    
    ¹ As noted in bb6278b ("avoid libcrypto.so requires", 2018-06-16)