tests/bundles/renames.hg

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

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