[Workbench] Fix unreliable repository browsing
Sometimes, after getting 401 from arv-git-httpd and asking the credential helper for a password, git exits non-zero instead of retrying the http request with the supplied password. Rare with git v1, common with git v2.
Example from build 1552
3) Error: RepositoriesTest#test_browse_using_arv-git-http: Capybara::ExpectationNotMet: expected to find text "Date: Tue Mar 18 15:55:28 2014 -0400" in "workbench:test Dashboard Projects Home Oh... fiddlesticks. Sorry, I had some trouble handling your request. `git fetch https://localhost:56182/active/foo.git +*:*` pid 13262 exit 128:" test/integration/repositories_browse_test.rb:50:in `block in <class:RepositoriesTest>' test/test_helper.rb:274:in `run'
Discussion on #5416 too.