Problems with updating Phab revision using arcanist

[Jan Stolarek]

Hi all,

I have a question to Phab gurus about using arcanist. I created a git branch, hacked a bit and 
then submitted a review using `arc diff`. Then I make a few more commits and want to update my 
revision. Running `arc diff` opens up an editor where I can create a new revision. Obviously, 
that's not what I want so I follow the suggestion from the comment in the editor to use 
`--update`:

# arc diff --update D202
Exception
Can't parse an empty diff!
(Run with --trace for a full exception trace.)

That's not what I want either. I'm left with manually specifying the base commit:

arc diff 1719c42

and that works. But that's not really convenient. Any ideas why neither `arc diff` nor `arc 
diff --update <rev>` don't work (I believe they should)?

Janek