Martin von Zweigbergk <martinvonz@google.com> [Wed, 21 Mar 2018 11:05:32 -0700] rev 37035
rebase: look up commit message to reuse outside of conclude[memory]node()
This was done the same way in both functions, so let's let the single
caller do it.
Differential Revision: https://phab.mercurial-scm.org/D2916