histedit: don't crash if commit message is empty
If the commit message is empty, histedit will crash before this patch
because it assumes that `summary.splitlines()` is non-empty. One of
our users at work ran into this crash for a commit that was created by
an internal system.
I don't think we have a good way of testing this because it's hard to
create a commit with an empty commit message. I've added a comment to
help prevent regressions.
Differential Revision: https://phab.mercurial-scm.org/D9325
edition = "2018"
max_width = 79
wrap_comments = true
error_on_line_overflow = true