feat: delete-branch multiple unique branch names completions

[johnpyp]

This PR improves git delete-branch bash and zsh completions by making them each accept multiple unique branch names.

The __gitex_branch_names_unique is identical to __gitex_branch_names (and corresponding in bash) except that:

• The added -F flag in zsh and the for loop in bash make it so that if a branch was already specified in the argument list, it doesn't get specified again.
• The COMPWORDS[@]:2 and ${words[2,-1]} stuff is there to make it so that the code doesn't include the delete-branch argument as one of the already deleted branches, which accounts for the admittedly very unlikely edgecase that delete-branch itself is a branch

[hyperupcall]

Thanks!