Better branch management in existing repositories

Detect 'IgnoreBranches' setting value to set for already existing repositories

when the value of the setting is not set
and depending of the number of Tfs remotes initialized.
if at least 2 tfs remotes, then we want to support branches
otherwise, ignoring them...
I don't really like persisting config as a side effect of, say `git tfs fetch`. Can this line go away?