Unfriendly errors can arise when creating initial entity version

[jonc125]

See https://rollbar.com/jonc125/Web-Lab/items/86 - user sees error

GitCommandError: Cmd('git') failed due to: exit code(128) cmdline: git reset --hard HEAD~ --
stderr: 'fatal: Failed to resolve 'HEAD~' as a valid revision.'
On page https://scrambler.cs.ox.ac.uk/entities/models/442/versions/new

Probably because a git error happened while adding or removing a file, and the hard_reset then fails because no commit yet exists. Not sure what exactly the user did to cause this.

[jonc125]

I figured out how to reproduce this while testing #286!

If you go through the process to create a new model, add a CellML file, enter an invalid tag name, and click 'create version' you'll get the error above.

So probably we just don't want to perform the hard reset if there isn't an existing commit.