Fix: git-tfs tries to authenticate against all TFS instances in the branch no matter how old they were in use and if they needed at all.

Our TFS instance recently changed URL and without this change it is required to filter-branch entire repository and change every git-tfs-id line in comments just to make git-tfs stop going to old URL to check. With it one should just keep config section for old branch and change the very last commit's comment (for each TFS branch). Also theoretically should save some time if several TFS instances are used as only specified with -i option is authenticated against.
Merge pull request #325 from git-tfs/fix-git-tfs-authenticates-against-all-tfs-instances

git-tfs tries to authenticate against all TFS instances in the branch no matter how old they are