Merge branch 'wm/complete-git-short-opts' into jch

The command line completion script (in contrib/) learned to
complete "git -<TAB>" to give single-letter options like "-C".

We do not complete single-letter options themselves.
cf. <xmqqo6onjfwl.fsf@gitster.g>

* wm/complete-git-short-opts:
  completion: complete "git -<TAB>" with short options

diff --cc contrib/completion/git-completion.bash
Simple merge