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
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true