indexer: start parsing input data immediately

Currently, the first call of git_indexer_stream_add adds the data to the
underlying pack file and opens it for later use, but doesn't start
parsing the already available data.
This means, git_indexer_stream_finalize only works if
git_indexer_stream_add was called at least twice. Kill this limitation
by parsing available data immediately.

src/indexer.c

@@ -313,8 +313,6 @@ int git_indexer_stream_add(git_indexer_stream *idx, const void *data, size_t siz
 		mwf = &idx->pack->mwf;
 		if (git_mwindow_file_register(&idx->pack->mwf) < 0)
 			return -1;
-
-		return 0;
 	}
 
 	if (!idx->parsed_header) {