fix chunk create return values

libgit2 docs look incorrect:
http://libgit2.github.com/libgit2/#HEAD/group/blob/git_blob_create_fromchunks

"When there is no more data to stream, callback should return 
1. This will prevent it from being invoked anymore."

Testing and a look at blob.c seems to confirm result should be 0 when there is no more data, -1 if error, and  otherwise length of data copied to buffer.