Use correct class for credentials_cb error

The first parameter of `git_error_set_str` is supposed to be an
error class, not the raw error code. The main consequence is that
the error message should show the class code instead of the error
code (which would usually just be GIT_ERROR).

Author: ehuss

src/remote_callbacks.rs

@@ -290,7 +290,7 @@ extern "C" fn credentials_cb(
 
             callback(url, username_from_url, cred_type).map_err(|e| {
                 let s = CString::new(e.to_string()).unwrap();
-                raw::git_error_set_str(e.raw_code() as c_int, s.as_ptr());
+                raw::git_error_set_str(e.class() as c_int, s.as_ptr());
                 e.raw_code() as c_int
             })
         });