Derive author from git var GIT_AUTHOR_IDENT

[aboks]

This is more accurate than retrieving the name and email using git config, as that does not take environment variables and command line flags into account.

Fixes #57

[shochdoerfer]

Just wondering, do you think it makes sense to keep the old behaviour in a separate action, as it may break existing setups?

[aboks]

Hmm 🤔 In my personal opinion that would not be necessary:

• git var has been around for ages (it existed at least in git 2.0.5, which was released back in 2014), so it seems unlikely that people are using a git version that does not support the var subcommand.
• It seems unlikely that people actually relied on the behavior that the action did not take command line flags and environment variables into account. It might be that the new version of the action rejects commits that would have been accepted previously, but in that case I think the user expects those commits to be rejected. Users probably use the action to validate the author metadata of their actual commits, not their git configuration.
• The package is still pre-1.0.0, so I would consider (minor) BC breaks like this to be acceptable. In the unlikely case users have trouble with the new version they can pin their current version and file an issue.

[shochdoerfer]

Thought the same but was not sure if I missed something.