adding credential fill to the executed commands in the CredentialHelper

[Black616Angel]

This PR adds the functionality of the "git credential fill" command to the git credential helper. This allows for better retrieval of credentials.

The PR is in 3 commits:

• repositioning a trailing "get" that got added to all commands, when it shouldn't have
• adding the aforementioned command in the function "config_helper" (this position is definitely debatable)
• checking if the returned username is empty, since "git credential fill" seems to always return something