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

hgext/histedit.py

@@ -533,7 +533,8 b' class histeditaction(object):'
             summary = cmdutil.rendertemplate(
                 ctx, ui.config(b'histedit', b'summary-template')
             )
-        summary = summary.splitlines()[0]
+        # Handle the fact that `''.splitlines() => []`
+        summary = summary.splitlines()[0] if summary else b''
         line = b'%s %s %s' % (self.verb, ctx, summary)
         # trim to 75 columns by default so it's not stupidly wide in my editor
         # (the 5 more are left for verb)