Use unambiguous algorithm to complete remote branches

The execution of `git branch --remotes --list` provides output like

  origin/HEAD -> origin/master
  origin/master

Each seperate word will be proposed as a completion argument. So, 4
values will be suggested instead of 2:

  origin/HEAD       - correct
  ->                - incorrect
  origin/master     - incorrect
  origin/master     - correct

That's why `git branch` is replaced with `git for-each-ref` command as
the last one always gives a correct list of remote branches.

completions/git-elegant.bash

@@ -26,7 +26,7 @@ _git_elegant() {
             accept-work|obtain-work)
                 local opts=(
                     ${gecops}
-                    $(git branch --remotes --list)
+                    $(git for-each-ref --format='%(refname:short)' refs/remotes)
                 )
                 COMPREPLY=(
                     $(compgen -W "${opts[*]}" -- ${cursor})