Do not crash if there is no commit

[trickypr]

Some people are experiencing crashes when building without a commit. e.g.

Error: Command failed with exit code 128: git rev-parse HEAD
fatal: not a git repository (or any of the parent directories): .git

We should:

1. Check if it is in a git repo
2. Check if there is a commit

If neither of those conditions are met, we should warn the user and use a bogus commit hash (e.g. 0000000 or something)