Merge branch 'ab/branch-sort' into maint

The implementation of "git branch --sort" wrt the detached HEAD
display has always been hacky, which has been cleaned up.

* ab/branch-sort:
  branch: show "HEAD detached" first under reverse sort
  branch: sort detached HEAD based on a flag
  ref-filter: move ref_sorting flags to a bitfield
  ref-filter: move "cmp_fn" assignment into "else if" arm
  ref-filter: add braces to if/else if/else chain
  branch tests: add to --sort tests
  branch: change "--local" to "--list" in comment