Add minimum commit message length for core.

Per https://meta.trac.wordpress.org/ticket/7784 an empty-message commit was made to develop.svn accidentally: https://core.trac.wordpress.org/changeset/59087

We’ve got a minimum-message-length enforced for plugins.svn & themes.svn, can that same check please be applied to at least develop.svn (or preferably all SVNs?)

#prio3 #svn