Introduce feature flag for credential.interactive

The changes in how the runner generates/manages the job's .git/config coupled with newer versions of Git handling of credential.interactive (https://github.com/git/git/blob/master/credential.c#L264) means that GIT_ASKPASS would not longer function as expected.

Since these newer versions of Git are not common yet, coupled with what seems like a number of ongoing changes to the runner shell packages, I want to make this an optional feature flag initially. Once I have had more time to test and gather feedback I'll explore making this the default moving forward.

It can enabled either by a runner admin through the configuration:

[general]
  ff_git_interactive = true

Alternatively, in a project's job/pipeline:

variables:
  FF_GIT_INTERACTIVE: true

Closes #195 (closed)