Everybody hated the pull behaviour of merging the first branch
listed on remotes/* file (or remote.*.fetch config) into the
current branch. This finally corrects that UI wart by
forbidding "git pull" without an explicit branch name on the
command line or branch.$current.merge for the current branch.
The matching change to git-clone was made to prepare the default
branch.*.merge entry for the primary branch some time ago.