more useful set-upstream

Is there a better way to achieve the functionality of this alias?

  set-upstream = "!sh -c 'current=$(git symbolic-ref HEAD); git branch
\"$current\" --set-upstream \"$1\"/$(basename $current)' -"

(forgive me if stupid gmail word wrapped that line)

This allows me to write

$ git set-upstream origin

instead of

$ git branch <current branch> --set-upstream origin/<current branch>

which is a pain to write for several reasons: I have to remember the current
branch name, that name might be long, and shell autocomplete doesn't know
about --set-upstream yet so won't complete the second occurrence.

Having a dedicated script (or git command) rather than an alias would allow
greater functionality like:
 - automatically pick the remote name if there's only one remote,
 - a flag to automatically set upstream on all branches (that having
   matching remotes)

I don't understand why all my branches haven't automatically had the right
upstream set anyway - am I missing some configuration of pull and push?