diff: no need for error when there's no HEAD (new Git repo)

[jorgeorpinel]

$ dvc version
WARNING: Unable to detect supported link types, as cache directory '.dvc/cache' doesn't exist. It is usually auto-created by commands such as `dvc add/fetch/pull/run/import`, but you could create it manually to enable this check.
DVC version: 0.83.0
Python version: 3.7.6
Platform: Darwin-19.3.0-x86_64-i386-64bit
Binary: False
Package: brew

$ git init
$ dvc init
$ echo foo > data
$ dvc diff
ERROR: failed to get diff - unknown Git revision 'HEAD'                 

Having any troubles? Hit us up at https://dvc.org/support, we are always happy to help!

^ It could simply not print anything, I think.

p.s. also not sure about the need for that WARNING in dvc version.

[jorgeorpinel]

p.p.s. it could even default to compare the wokspace vs nothingness in this case? I.e. show all changes (as if there was a HEAD that did not have any of the current files).

I'd check what git diff does.