set-upstream utility

would anyone be interested in a utility that does this? i can make a PR if so.

``` sh
#!/usr/bin/env sh
# git-pushup
#   Pushes the current branch via --set-upstream.
#   It's a shortcut for `git push --set-upstream origin <current branch>`.

remote=$1
[ -z $remote ] && remote=origin

git push --set-upstream $remote `git rev-parse --abbrev-ref HEAD`
```
