argv[0] could use some sanity checks

[alexandruavadanii]

git vanity-sha test will spend a lot of cycles before bailing out without a match, when it could have simply complained the required pattern is not valid hex.
This should be easy to fix, but I'm not fluent in Ruby, so I'll refrain from submitting a PR.

[misternu]

Fail early if:

  target_prefix =~ /^[a-f0-9]+$/

?

[mattbaker]

This should be easy to fix, but I'm not fluent in Ruby, so I'll refrain from submitting a PR.

No need to be fluent! Go for it! If there was one place to try submitting a Ruby PR it's this one