Differentiate credential files and simplify structure.

This MR attempts to accomplish a small handful of related improvements that will hopefully provide a fix for the reported issue (#35) as well as improve the overall structure.

• .credentials/pass-<jobid> has been relocated to avoid potentially conflicting with other files/configs.
• $\'git\' "config" "--local" "credential.helper" "\"\""\n is now appended to the configuration
• Rely upon executor environment in the creation/identification of askpass.

This still relies upon: https://github.com/git/git/commit/24321375cda79f141be72d1a842e930df6f41725

Long term there is now a core.askpass configuration option but we should further test and verify that target deployments have access to newer version of Git.