Fix summary for merge commits
Fix summary for merge commits

Problem before was that I was using 'git log' of the commit and
its parent, which doesn't work as I intended when there's more than
one parent. Instead, use 'git show -s', which is helpfully not
documented in the manpage for git-show, but instead the manpage for
git-shortlog

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome