filter: internal git_buf filter handling function Introduce `git_filter_list__convert_buf` which behaves like the old implementation of `git_filter_list__apply_data`, where it might move the input data buffer over into the output data buffer space for efficiency. This new implementation will do so in a more predictible way, always freeing the given input buffer (either moving it to the output buffer or filtering it into the output buffer first). Convert internal users to it.