Fix the gpg_sign commit option

[jcouball]

Signed-off-by: James Couball jcouball@yahoo.com

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Description

When support for the gpg_sign option was added to the Git::Lib#commit in #518, it did not actually work properly for the case of the key being passed into the commit command.

This PR does the following:

• Correctly allows specification of a GPG Key in the gpg-sign option as follows:

key_id = '0A46826A'
g.commit('message', gpg_sign: key_id)

• Add tests to make sure the commit command line is built correctly for the two use cases of gpg_sign.
• Add examples in the README.md

This fix also fixes the warning introduced in #518 the was output by Ruby when requiring the git gem:

$ irb
3.0.1 :001 > require 'git'
/Users/couballj/.rvm/gems/ruby-3.0.1/gems/git-1.9.0/lib/git/lib.rb:663: warning: string literal in condition
 => true
3.0.1 :002 >