Auto merge of #2584 - alexcrichton:auth-more, r=brson

Correctly attempt multiple usernames with libgit2

This commit corrects the logic for attempting multiple usernames with libgit2.
There is a restriction that for each authentication seession you can only
authenticate with one ssh username, but we were attempting multiple as part of
the same sesssion. Instead restart authentication whenever we try a new username.

Closes #2399