Adds the ability to `--gpg-sign` new commits to the upstream

[edvinassabaliauskas]

• Added new option --gpg-sign to branch, pull, push commands.
• Also fixes #613 as it is related for the feature to work correctly.

[kuwannet]

I’ve been playing with this and have some comments…

1. It would be good to honor whether commit.gpgsign=true has been set via git config and automatically pass -S or --gpg-sign to the commands that need it. Per the git documentation:

   To configure your Git client to sign commits by default for a local repository, in Git versions 2.0.0 and above, run git config commit.gpgsign true. To sign all commits by default in any local repository on your computer, run git config --global commit.gpgsign true.

   When you have commit.gpgsign=true set and are used to having commits always signed automatically, it’s easy to forget to add the option to the git subrepo … commands.

2. It would also be nice to use the -S option as a synonym for --gpg-sign as is supported by git itself:

   -S[<keyid>], --gpg-sign[=<keyid>], …
       GPG-sign commits. The keyid argument is optional and defaults to the committer identity; if specified, it must be stuck to the option
       without a space…