Directly run a diff on two blobs.
Compared to a file, a blob lacks some contextual information. As such,
the git_diff_file given to the callback will have some fake data; i.e.
mode will be 0 and path will be NULL.
NULL is allowed for either old_blob or new_blob and will be treated
as an empty blob, with the oid set to NULL in the git_diff_file data.
Passing NULL for both blobs is a noop; no callbacks will be made at all.
We do run a binary content check on the blob content and if either blob
looks like binary data, the git_diff_delta binary attribute will be set
to 1 and no call to the hunk_cb nor line_cb will be made (unless you pass
GIT_DIFF_FORCE_TEXT of course).
Treat old blob as if it had this filename; can be NULL
Treat new blob as if it had this filename; can be NULL
Payload passed to each callback function
0 on success, non-zero callback return value, or error code