We take in a possibly partial ID by taking a length and working off of
that to figure out whether to just look up the object or ask the
backends for a prefix lookup.

Unfortunately we've been checking the size against `GIT_OID_HEXSZ` which
is the size of a *string* containing a full ID, whereas we need to check
against the size we can have when it's a 20-byte array.

Change the checks and comment to use `GIT_OID_RAWSZ` which is the
correct size of a git_oid to have when full.

There are some combination of objects and target types which we know
cannot be fulfilled. Return EINVALIDSPEC for those to signify that there
is a mismatch in the user-provided data and what the object model is
capable of satisfying.

If we start at a tag and in the course of peeling find out that we
cannot reach a particular type, we return EPEEL.

The brace in the check for peel's return was surrounding the wrong
thing, which made 'error' be set to 1 when there was an error instead of
the error code.

This finds a short id string that will unambiguously select the
given object, starting with the core.abbrev length (usually 7)
and growing until it is no longer ambiguous.