README.md

The command line completion (in contrib/) completed "git branch -d"
with branch names, but "git branch -D" offered tagnames in addition,
which has been corrected.  "git branch -M" had the same problem.

* jk/complete-branch-force-delete:
  doc/git-branch: fix awkward wording for "-c"
  completion: handle other variants of "branch -m"
  completion: treat "branch -D" the same way as "branch -d"