Support the commit --no-gpg-sign flag

[bradleybuda]

Description

Add the ability to send the --no-gpg-sign option to git. This is the default behavior in a "vanilla" git install, but if gpg signing is configured elsewhere (i.e. in ~/.gitconfig) then you can use this option to override and disable it for a particular commit.

Triggered by gpg_sign: false - see test and README updates.

[jcouball]

@bradleybuda I like this PR, but I request the following change:

Instead of passing gpg_sign as false, I think it would be more consistent with the rest of the API to pass no_gpg_sign. You can see examples of this for the following:

• git commit --no-verify
• git merge --no-commit
• git merge --no-ff
• git revert --no-edit

The design of the API mimics what you would pass on the command line. Like it or not, that is the design we have today.

[jcouball]

I have fixed the Ruby 2.7 windows-latest build in #591

[bradleybuda]

Thanks for the comment @jcouball, updated the patch. I hadn't noticed that there were some other --no--prefixed flags already in the code so agree that this should be consistent.

[jcouball]

Looks great! Thanks for the thorough tests!