util: stop overwriting sha1, overwrite _fastsha1 instead
Some modules (like revlog) would import util.sha1 as _sha1. This
defeats the purpose of having util.sha1 overwrite itself with a faster
version -- revlog would end up always calling the slow version. By
always delegating to util._fastsha1 we avoid this at the cost of an
extra (but unconditional) indirection.