Correct a typo in magit-list-remote-branch-names

match-string want an integer as first argument.

Replacing the "and" by a "when" because the second value is not a truth
value.

Added also some test on the different function that should list branch.
Merge pull request #1407 from vanicat/t/typo-in-magit-list-remote-branch-names2

Correct a typo in magit-list-remote-branch-names