Don't replace slashes in git URLs. Fixes JENKINS-13243

otherwise git://github.com/user/repo.git becomes git:\github.com\user\repo.git 

see https://issues.jenkins-ci.org/browse/JENKINS-13243?focusedCommentId=178712&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-178712