Merge branch 'sp/describe'

* sp/describe:
  Use git-describe --exact-match in bash prompt on detached HEAD
  Teach git-describe --exact-match to avoid expensive tag searches
  Avoid accessing non-tag refs in git-describe unless --all is requested
  Teach git-describe to use peeled ref information when scanning tags
  Optimize peel_ref for the current ref of a for_each_ref callback