magit-insert-remote-branches: Account for HEAD

When running `git branch -vvr', cloned repos have a line like below.

    origin/HEAD               -> origin/master

This is not matched by `magit-wash-branch-line-re', leading to an error
when `magit-bind-match-strings' is called.

Now, `magit-bind-match-strings' is called only when a match is found.
Merge pull request #1426 from kyleam/fix-show-refs

magit-insert-remote-branches: Account for HEAD