Expand Git repo URL normalization

[merenbach]

To resolve #138:

1. Lowercase repo URLs before they are hashed for purposes of secret creation. This will break all existing repos that have ANY uppercase characters, thanks to hashing differences.
2. Ensure that all added repos have a .git suffix during normalization. This will break all existing repos that were added without a .git suffix, thanks to hashing differences.
3. Consider repo URLs to be SSH repos if they start with git@ (existing behavior) or if they begin with ssh:// (new behavior). All SSH prefixes will be stripped at the end of the normalization process.
4. Normalization will now return an empty string if the URL does not parse. Since rootless GitHub URLs won't parse as-is, an SSH scheme is added to these, then stripped at the end.
5. Refactor and add additional tests for Git- and Git-repo-related functionality in the util/git module.

[jessesuen]

Can we add some tests where the input is missing the .git suffix, and added after normalization?

[jessesuen]

Thanks for the thorough job and tests!

[jessesuen]

Can we add one where "https://github.com/argoproj/argo-cd.git" also maps to "repo-argo-cd-821842295"