Supports multiple credential helpers nicely

[keiko713]

- Do you want to request a feature or report a bug?

bug

- What is the current behavior?

netlify lm:install will make the git pull or anything impossible with folks using https git + osxkeychain helper

- If the current behavior is a bug, please provide the steps to reproduce.

After running netlify lm:install, it'll add the following two things:

## to global config

# This next lines include Netlify's Git Credential Helper configuration in your Git configuration.
[include]
  path = /Users/keikooda/.netlify/helper/git-config

## inside of git-config

# The first line resets the list of helpers so we can check Netlify's first.
[credential]
  helper = ""

[credential]
  helper = netlify
  useHttpPath = true

If you're using git-credential-osxkeychain helper:

1. it's not added in git-config file (because currentCredentials is only checking --global and osxkeychain is --system), then things like git pull fails because
2. even if we manually add it, it still complains. it turned out that it SOMEHOW set the credential as useHttpPath=true even it's not specifically said, so oskeychain helper was saying "I don't have credential, sorry"

Seems like the solution here is to add all helpers in git-config file with useHttpPath=false (unless it actually has true setting).

- What is the expected behavior?

netlify lm:setup won't disturb other credential helpers

- Please mention your node.js, and operating system version.

tried with osx, I think node version is unrelated to this issue

[keiko713]

@calavera can you take a look this when you have time?