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