Don't wipe out b:airline_head if fugitive finds the head

- Fugitive can edit virtual files that represent objects in git; while
  editing these files, it still reports on the HEAD of the relevant
  repo, which should be displayed as the branch (rather than having the
  branch section disappear)
Merge pull request #577 from qstrahl/master

Don't wipe out b:airline_head if fugitive finds the head