histedit: use new function for getting first line of a string
This fixes a crash you can run into if you enter a commit message
that's just a "newline-like" byte, like a form feed byte (`hg ci -m
\x0f` in Fish). That bug is the motivation for this series.
Differential Revision: https://phab.mercurial-scm.org/D12405
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true