Flake: "Git commands for CompositeGitStrategy ... prefers user-configured credential helpers if present"

[smashwilson]

Test failure seen on Travis on a Mac instance.

 1) Git commands for CompositeGitStrategy made of [GitShellOutStrategy] https authentication prefers user-configured credential helpers if present:
     git fetch mock master in /private/var/folders/my/m6ynh3bn6tq06h7xr3js0z7r0000gn/T/git-fixture-11743-4030-13lithw.6bnws714i exited with code 128
stdout: 
stderr: 
  Error
      at new GitError (/Users/travis/build/atom/github/git-shell-out-strategy.js:26:18)
      at /Users/travis/build/atom/github/git-shell-out-strategy.js:181:23
      at next (<anonymous>)
      at step (/Users/travis/build/atom/github/git-shell-out-strategy.js:16:1)
      at /Users/travis/build/atom/github/git-shell-out-strategy.js:16:1