Merge branch 'rs/wt-status-detached-branch-fix'

"git status --branch --short" accessed beyond the constant string
"HEAD", which has been corrected.

* rs/wt-status-detached-branch-fix:
  wt-status: use skip_prefix() to get rid of magic string length constants
  wt-status: don't skip a magical number of characters blindly
  wt-status: avoid building bogus branch name with detached HEAD
  wt-status: exit early using goto in wt_shortstatus_print_tracking()
  t7060: add test for status --branch on a detached HEAD
