Use credentials in install_git when using git2r

[jimhester]

Fixes #106

[jimhester]

I think the main question with this is the behavior if you try to use credentials with an non git2r backend. Right now it errors in this case.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@6fbeeec). Click here to learn what that means.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##             master     #146   +/-   ##
=========================================
  Coverage          ?   86.15%           
=========================================
  Files             ?       29           
  Lines             ?     1690           
  Branches          ?        0           
=========================================
  Hits              ?     1456           
  Misses            ?      234           
  Partials          ?        0

Impacted Files	Coverage Δ
R/install-git.R	89.77% <88.88%> (ø)

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[gaborcsardi]

I think the main question with this is the behavior if you try to use credentials with an non git2r backend. Right now it errors in this case.

I think that's fine.

[jimhester]

Thanks for the review!