Read an object from the database, given a prefix of its identifier.
This method queries all available ODB backends trying to match the 'len' first hexadecimal characters of the 'short_id'. The remaining (GIT_OID_SHA1_HEXSIZE-len)*4 bits of 'short_id' must be 0s. 'len' must be at least GIT_OID_MINPREFIXLEN, and the prefix must be long enough to identify a unique object in all the backends; the method will fail otherwise.
The returned object is reference counted and internally cached, so it should be closed by the user once it's no longer in use.
the length of the prefix
0 if the object was read, GIT_ENOTFOUND if the object is not in the database. GIT_EAMBIGUOUS if the prefix is ambiguous (several objects match the prefix)