use commit hash as git branch with detached HEAD

Abbreviated to 7 characters to match `git rev-parse --short HEAD`.
Merge pull request #580 from andrewgregory/detached_head

use commit hash as git branch with detached HEAD
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)