Why doesn't git allow me to safely delete a branch?

Question

I needed to merge two branches -- second into first and then get rid of second. Here's what I did:

• git cloned the project to get a fresh copy
• git checkout --track origin/second, made some changes, and committed
• git checkout --track origin/first, made some changes, and committed
• git merge second (git says "merge made by recursive")
• git branch -d second

Then git says:

$ git branch -d second
warning: not deleting branch 'second' that is not yet merged to
         'refs/remotes/origin/second', even though it is merged to HEAD.
error: The branch 'second' is not fully merged.
If you are sure you want to delete it, run 'git branch -D second'.

Why is this happening? I've never gotten this message after a merge before. The merge worked just fine, no conflicts. How do I safely delete the second branch?

Answer 1

Based on my experiments and @knittl's and @twalberg's comments, it seems that git just wanted me to push my changes to the second branch before deleting it.

I did:

$ git checkout second
$ git push origin second
$ git checkout first
$ git branch -d second

which worked without warnings.