Add ref completion to git-missing.

I find useful to get the ref completion in the git-missing command,
the behavior is the *like* the one on 'git log --pretty' it use the
'git for-each-ref' command to provide shortname of :
* All local branches.
* All remote/branches.
* All tags.
* The stash.

The output can be a bit huge, but provide a standard git / bash completion.

etc/bash_completion.sh

@@ -57,6 +57,11 @@ _git_ignore(){
   esac
 }
 
+_git_missing(){
+    # Suggest all known refs
+    __gitcomp "$(git for-each-ref --format='%(refname:short)')"
+}
+
 _git_refactor(){
   __git_extras_workflow "refactor"
 }