with git-next, git pull --rebase can print out "fatal: No such ref: ''" for branches without an upstream

With git-next, where git pull --rebase can print out "fatal: No such ref: ''" if git pull --rebase is run on branches without an upstream.

With git at b139ac2589b15d55cd9fa5c6957da44b150d0737, the following commands demonstrate the problem:

git init repo1 cd repo1 touch a; git add a; git commit -m "a" cd .. git clone repo1 repo2 cd repo2 git config remote.origin.fetch refs/heads/master:refs/remotes/origin/master git checkout -b test git pull --rebase

This results in the following output:

fatal: No such ref: '' Current branch test is up to date.

So the pull --rebase looks like it works, but it prints out a spurious fatal error.

I've managed to bisect this down to https://github.com/gitster/git/commit/48059e405028ebf8a09c5a9aede89dfb460cce98. Looks like get_remote_merge_branch is called without arguments, and it returns an empty string. This string is passed as-is to git merge-base, which causes the error.