use git describe for naming version in pure git clones

Patch by: Ramiro Polla <ramiro <DOT> polla  gmail <DOT> com>

Originally committed as revision 26394 to svn://svn.ffmpeg.org/ffmpeg/trunk

version.sh

@@ -18,7 +18,7 @@ fi
 
 # check for git short hash
 if ! test $revision; then
-    revision=$(cd "$1" && git log -1 --pretty=format:%h 2> /dev/null)
+    revision=$(cd "$1" && git describe --always 2> /dev/null)
     test $revision && revision=git-$revision
 fi