WIP: LibGit2 support for Git credential helpers

[omus]

Refactored the LibGit2 credential callback to add support for Git credential helpers. Since the credential callback code was mostly untested I also wrote tests for this code and fixed several issues that were discovered by the new tests. Changes include:

• Support LibGit2 configuration iterators
• Use LibGit2 callback payload to keep track of state
• Add Git credential helper support to HTTPS authentication
• Avoid sending invalid credentials multiple times to remote servers
• Support use of EOF character (^D) to abort credential prompt
• Limit amount of re-prompts
• Revise when cached credentials are accepted or rejected
• Testing framework which automates credential prompt responses and does not require external servers

Additional work to do:

• Make tests compatible with Windows Disable tests for Windows
• Disable git credential helper tests on systems where command-line git isn't installed
• Add more tests for credential approval/rejection
• Add more git credential tests
• Document credential prompt abort using ^D

[ararslan]

Is this actually release blocking for 0.6? If not, 1.0 might be the more appropriate milestone. (cc @tkelman)

[omus]

I can see the argument for not having this be blocking for 0.6. However, this work does include numerous fixes and tests I would like to see included in a 0.6 release.

Additionally, I require the git credential helper feature for running a GitLab CI with support private package dependencies on Julia 0.6

[tkelman]

If the git credential helper relies on command-line git, then I don't think it should live in base. Can you separate the fixes, tests, and features here? This is a pretty big PR to be opening as we're basically one build-fix away from feature freeze.

[omus]

I broke up the PR into smaller commits. Unfortunately GitHub isn't showing the commits in the right order. Here's a listing in the correct order:

1. Improve scp-like syntax parsing (1f3b39b) (bugfix)
2. Refactored LibGit2 credential callback (6ea2a68) (bugfix)
3. Limit amount of re-prompts (73dc088) (feature)
4. Use of EOF character (^D) to abort cred prompt (c661d60) (feature)
5. Add LibGit2 support for config iterator (a462aeb) (git credential helper)
6. Add GitConfig into RemotePayload (4ac043d) (git credential helper)
7. Add framework for Git credential helper support (409bb3b) (git credential helper)
8. Add Git credential helper support for HTTPS (b0bf68d) (git credential helper)

[omus]

I'll be breaking this PR up into multiple PRs to make things faster and easier to review.

[omus]

Removing 0.6.0 milestone as this is no longer critical. Details in #20769

[omus]

With the merger of #23824 this PR can now be retired.