Merge branch 'rj/complete-reflog'

The command line completion script (in contrib/) learned to
complete "git reflog" better.

* rj/complete-reflog:
  completion: reflog subcommands and options
  completion: factor out __git_resolve_builtins
  completion: introduce __git_find_subcommand
  completion: reflog show <log-options>
  completion: reflog with implicit "show"

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

diff --cc t/t9902-completion.sh
Simple merge