mkrevision: Append "-dirty" if git working copy is modified
mkrevision: Append "-dirty" if git working copy is modified

- Add "-dirty" to the generated revision string if the git working copy has
been modified from the HEAD revision.  Under Subversion, svnversion adds "M"
in this case, but under git we have to do this ourselves.

cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr,cmusatyalab/openisr