Shell completion for HEAD ref ignores tags

[jasonkarns]

Attempted to do:

git imerge start --name=rc1 --first-parent v<tab> to get shell completion of the HEAD ref. (All tags are prefixed with v.) However, the completion at that point only seems to include branches.

[mhagger]

All of the bash completion was implemented by @ralfth. If he or somebody else is willing to help out with this, that would be great (I haven't ever implemented bash completion code myself).