Support --amend

[FRidh]

There are quite some actions out there that create commits. Unfortunately, they are not signed. Adding support for --amend should, if I am correct, make it possible to sign commits created by other actions.

Note I do not know whether ghcommit supports that.

[npanuhin]

I also really need the ability to amend a commit (other/custom parameters for the git commit would also be nice)

[joemiller]

This will depend on whether or not the createCommitOnBranch graphql api supports amending existing commits. From a glance at the docs I do not think that's the case. https://docs.github.com/en/graphql/reference/mutations#createcommitonbranch

I also searched that page for "amend" and came up with nothing :|

If anyone finds a method for amending I'd be happy to take a look at implementing it in https://github.com/planetscale/ghcommit

This may require a feature request to github's graphql api