It seems like as of D11995 it is not possible to work in a directory with untracked files. `arc diff` no longer allows to continue until all files are added or ignored
Related to T7521
Discussion from IRC:
```[12:06] <sshannin> Does anybody know if aroravishal's comment at the bottom of D11995 is desired behavior?
[12:06] <phabot> D11995: Add untracked files to commit using prompt - https://secure.phabricator.com/D11995
[12:06] <sshannin> Specifically, arcanist basically no longer supports untracked files, which is a bit of a nuisance
[12:13] <adrelanos> Hi! Does phabricator provide a sitemap[.xml] [for search engines]?
[12:16] <cspeckmim> sshannin: when answering No for adding the untracked files, it stops the process instead of not adding the untracked files?
[12:23] <sshannin> correct
[12:23] <sshannin> cspeckmim: It errors out with "Usage Exception: You can not continue with untracked changes. Add them, discard them, or mark them as ignored before proceeding."
[12:45] == qchris has changed nick to qchris_away
[12:46] == Lcawte|Away has changed nick to Lcawte
[13:15] <avive> I hope not, I always keep untracked files around...
[13:22] <sshannin> yeah, likewise, but I'm getting them forced on me now
[13:25] <avive> :(
[13:27] <sshannin> not sure if it's just an unintended side-effect of that change or if it's intentional
```