Merge branch 'jc/strbuf-branchname-fix'

"git merge @{-1}~22" was rewritten to "git merge frotz@{1}~22"
incorrectly when your previous branch was "frotz" (it should be
rewritten to "git merge frotz~22" instead).

* jc/strbuf-branchname-fix:
  strbuf_branchname(): do not double-expand @{-1}~22