Simplify delete-merged-branches

This will delete branches merged into the current branch.

This uses `-d` and is therefore safer than the current script (which uses `-D`).
IMO, this mitigates the need to prompt.
git-changelog: don't output a trailing space after the date

This has been driving me and others crazy for forever.
/cc @guille @MatthewMueller
Merge pull request #201 from TooTallNate/patch-1

git-changelog: don't output a trailing space after the date
Merge pull request #199 from tlvince/refactor/git-delete-merged-branches

Simplify delete-merged-branches