gitmail: add short sha1 to subject line
gitmail: add short sha1 to subject line

Add the short sha1 to the end of the subject line for MUAs that thread by
subject instead of References/In-Reply-To headers.

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