stringutil: try to avoid running `splitlines()` only to get first line It's wasteful to call `splitlines()` and only get the first line from it. However, Python doesn't seem to provide a built-in way of doing just one split based on the set of bytes used by `splitlines()`. As a workaround, we do an initial split on just LF and then call `splitlines()` on the result. Thanks to Joerg for this suggestion. I didn't bother to also split on CR, so users with old Mac editors (or repos created by such editors) will not get this performance improvement. Differential Revision: https://phab.mercurial-scm.org/D12413