Support ssh signing [fixes #123]

[jeremy-w]

This mirrors the logic in git's sign_buffer_ssh() function in order to support signing commits with ssh keys.

The test assumes an ssh-agent is running. It heavily mirrors test_gpgsign, but omits the tests for argument/config handling around whether or not to sign.

[fixes #123]

[charettes]

If it can be of any help, I've been using this feature branch to successfully sign my commits for the past few days without issues.

Thanks for the patch @jeremy-w 🙇

[olmokramer]

I tried this patch but couldn't get it to work because I'm using gpg.ssh.defaultKeyCommand to get the key to use for signing, but it is not used in this PR. I'm getting the following error:

Traceback (most recent call last):
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/bin/.git-revise-wrapped", line 9, in <module>
    sys.exit(main())
             ^^^^^^
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/lib/python3.11/site-packages/gitrevise/tui.py", line 264, in main
    inner_main(args, repo)
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/lib/python3.11/site-packages/gitrevise/tui.py", line 244, in inner_main
    staged = repo.index.commit(message=b"<git index>")
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/lib/python3.11/site-packages/gitrevise/odb.py", line 904, in commit
    return self.repo.new_commit(self.tree(), [parent], message)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/lib/python3.11/site-packages/gitrevise/odb.py", line 317, in new_commit
    body += self.sign_buffer(body + body_tail)
            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/nix/store/w12qszjhngb8a4fi7h8jjhyllaq3as3c-python3.11-git-revise-0.7.0/lib/python3.11/site-packages/gitrevise/odb.py", line 347, in sign_buffer
    key_file_context_manager = open(key_id, "rb")
                               ^^^^^^^^^^^^^^^^^^
FileNotFoundError: [Errno 2] No such file or directory: b'Olmo Kramer <redacted@example.com>'