Rework the detection of the svn or git-svn revision
Rework the detection of the svn or git-svn revision

* Don't check whether .git or .svn exist, and don't rely on which to
  find the git or svn binaries, just try getting the revision directly
* Redirect the relevant stderr outputs to /dev/null to avoid warnings
  when git or svn are missing
* Use the modern "git foo" syntax instead of "git-foo"


git-svn-id: 29c1264a5cf5e3532df57b06678e7347571c1a3c@2932 f1ba3bf5-cb5c-402b-92a9-7c6bdc83a356

ananos/open-mx,ananos/open-mx,ananos/xen2mx,ananos/open-mx,ananos/xen2mx,ananos/xen2mx,ananos/xen2mx