dvc: preserve desc when re-add/import[-url]/run'ing

[efiop]

Currently desc is not preserved if you decide to update your dataset with dvc add. E.g.

#!/bin/bash                             
                                        
set -e                                  
set -x                                  
                                        
rm -rf myrepo                           
mkdir myrepo                            
cd myrepo                               
git init                                
dvc init                                
                                        
echo foo > foo                          
dvc add foo --desc 'my data description'
cat foo.dvc                             
# outs:
# - md5: d3b07384d113edec49eaa6238ad5ff00
#   size: 4                              
#   path: foo                            
#   desc: my data description            
                                        
echo foo >> foo                         
dvc add foo                             
cat foo.dvc
# outs:
# - md5: d3b07384d113edec49eaa6238ad5ff00
#   size: 4                              
#   path: foo