dvc: add support for dvc diff command

[django-kz]

Some summary statistics regarding the folder/file.
How many files were added/deleted/modified. How the size was changed.
TO DO things:
1)To add support for detecting diff after dvc move
command.
2)To add unit tests.

[shcheklein]

b_commit = ... a_ref looks pretty strange :) That's why I would probably just name params just --ref.

[django-kz]

@shcheklein, @efiop Guys take a look, and we still need to discuss the CLI syntax.

[efiop]

@django-kz I don't think your patch works:

#!/bin/bash               
                          
set -e                    
set -x                    
                          
rm -rf myrepo             
mkdir myrepo              
cd myrepo                 
git init                  
dvc init                  
                          
git add .dvc              
git commit -m init        
git tag v0                
                          
mkdir dir                 
echo foo > dir/foo        
dvc add dir               
                          
git add dir.dvc .gitignore
git commit -m dir         
                          
dvc diff v0               

prints

dvc diff from 3faa54e3c1b68cc890ab2192214041a44408d7ca to baad677e62d8b8d44e783dea4314a1099a0ecfd0 
                                                                                                   
diff for 'dir'                                                                                     
-dir with md5 aa5f690fab1a8e34ad009d3de2ad38cb.dir                                                 
                                                                                                   
0 files not changed, 0 files modified, 0 files added, 1 file deleted, size was decreased by 4 Bytes

which doesn't seem right. Or am I missing something?

[django-kz]

@efiop Actually, here it shows what changes happened from your current ref and specified one, I mean I expect it to work that way, so it is not a bug, it is a bad design )

[efiop]

@django-kz Also, please take a look at codeclimate report. It has some really good points in it.

[efiop]

Thanks! 🎉 Mind also adding a simple command reference to dvc.org? E.g. see iterative/dvc.org@6406bb8 as an example. Or, at least add an issue for it to dvc.org issues.

[efiop]

@django-kz Also, please don't forget to at least add those "TODO"s as issues to https://github.com/iterative/dvc/issues .