rebase: use @{upstream} if no upstream specified

'git rebase' without arguments is currently not supported. Make it
default to 'git rebase @{upstream}'. That is also what 'git pull
[--rebase]' defaults to, so it only makes sense that 'git rebase'
defaults to the same thing.

Defaulting to @{upstream} will make it possible to run e.g. 'git
rebase -i' without arguments, which is probably a quite common use
case. It also improves the scenario where you have multiple branches
that rebase against a remote-tracking branch, where you currently have
to choose between the extra network delay of 'git pull' or the
slightly awkward keys to enter 'git rebase @{u}'.

The error reporting when no upstream is configured for the current
branch or when no branch is checked out is reused from git-pull.sh. A
function is extracted into git-parse-remote.sh for this purpose.

Helped-by: Yann Dirson <ydirson@altern.org>
Helped-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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