Revert "object: correct the expected ID size in prefix lookup"

Revert "object: correct the expected ID size in prefix lookup"

This reverts commit 969d4b703c910a8fd045baafbcd243b4c9825316.

This was a fluke from Coverity. The length to all the APIs in the
library is supposed to be passed in as nibbles, not bytes. Passing it as
bytes would prevent us from parsing uneven-sized SHA1 strings.

Also, the rest of the library was still using nibbles (including
revparse and the odb_prefix APIs), so this change was seriously breaking
things in unexpected ways. ^^