BUG: git pull --rebase on detatched head prints a lot of usage warnings

Heya,

sverre@laptop-sverre:~/code/Melange
$ git pull --rebase
usage: git merge-base [-a|--all] [--octopus] <commit> <commit>...
   or: git merge-base --independent <commit>...

    -a, --all             output all common ancestors
    --octopus             find ancestors for a single n-way merge
    --independent         list revs not reachable from others

usage: git merge-base [-a|--all] [--octopus] <commit> <commit>...
   or: git merge-base --independent <commit>...

    -a, --all             output all common ancestors
    --octopus             find ancestors for a single n-way merge
    --independent         list revs not reachable from others

usage: git merge-base [-a|--all] [--octopus] <commit> <commit>...
   or: git merge-base --independent <commit>...

    -a, --all             output all common ancestors
    --octopus             find ancestors for a single n-way merge
    --independent         list revs not reachable from others

[... many more snipped ...]

You are not currently on a branch, so I cannot use any 'branch.<branchname>.merge' in your configuration file. Please specify which remote branch you want to use on the command line and try again (e.g. 'git pull <repository> <refspec>'). See git-pull(1) for details.

There are 127 helpful messages in total.

Output with GIT_TRACE set:

trace: exec: 'git-pull' '--rebase' trace: run_command: 'git-pull' '--rebase' trace: built-in: git 'rev-parse' '--git-dir' trace: built-in: git 'rev-parse' '--is-inside-work-tree' trace: built-in: git 'rev-parse' '--show-toplevel' trace: built-in: git 'ls-files' '-u' trace: built-in: git 'symbolic-ref' '-q' 'HEAD' trace: built-in: git 'config' '--bool' 'branch..rebase' trace: built-in: git 'rev-parse' '-q' '--verify' 'HEAD' trace: built-in: git 'rev-parse' '--verify' 'HEAD' trace: built-in: git 'update-index' '-q' '--ignore-submodules' '--refresh' trace: built-in: git 'diff-files' '--quiet' '--ignore-submodules' trace: built-in: git 'diff-index' '--cached' '--quiet' '--ignore-submodules' 'HEAD' '--' trace: built-in: git 'rev-parse' '-q' '--git-dir' trace: built-in: git 'symbolic-ref' '-q' 'HEAD' trace: built-in: git 'config' '--get' 'branch..remote' trace: built-in: git 'symbolic-ref' '-q' 'HEAD' trace: built-in: git 'for-each-ref' '--format=%(upstream)' trace: built-in: git 'rev-parse' '-q' '--verify' 'refs/remotes/origin/tip' trace: built-in: git 'rev-list' '-g' 'refs/remotes/origin/tip' trace: built-in: git 'merge-base' '927b640697cf707a8d3d26b39c926be052c3aa7c' trace: built-in: git 'merge-base' '8281385d7b5132a51ae1f3ad515f0220478c7f40'

Tested on current next.