Using DESTINATION_GITHUB_USERNAME instead of USER_NAME for cloning and pushing

[gabrieleara]

I feel that using variable USER_NAME in the git clone and git push commands is not correct, since it is used also to set the user.name property using 'git config`.

Typically, this property contains real user information (name and surname and such), which differs from the GitHub username.
For example, I typically set it to "Gabriele Ara".

This creates a bunch of problems when using this option in this action, because:

• it has spaces in it (which git/curl does not like at all);
• it does not correspond to any github username (my username is close but not quite the same).

I feel that simply changing it to DESTINATION_GITHUB_USERNAME would solve the issue.

[cpina]

Thanks for the PR - I will consider it but first I want to write some tests to avoid regressions in the current version. That's why I haven't done it yet (and lack of time for doing them :-) )