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>

Documentation/config.txt

@@ -646,7 +646,7 @@ branch.<name>.remote::
 
 branch.<name>.merge::
 	Defines, together with branch.<name>.remote, the upstream branch
-	for the given branch. It tells 'git fetch'/'git pull' which
+	for the given branch. It tells 'git fetch'/'git pull'/'git rebase' which
 	branch to merge and can also affect 'git push' (see push.default).
 	When in branch <name>, it tells 'git fetch' the default
 	refspec to be marked for merging in FETCH_HEAD. The value is

Documentation/git-rebase.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git rebase' [-i | --interactive] [options] [--onto <newbase>]
-	<upstream> [<branch>]
+	[<upstream>] [<branch>]
 'git rebase' [-i | --interactive] [options] --onto <newbase>
 	--root [<branch>]
 
@@ -21,6 +21,12 @@ If <branch> is specified, 'git rebase' will perform an automatic
 `git checkout <branch>` before doing anything else.  Otherwise
 it remains on the current branch.
 
+If <upstream> is not specified, the upstream configured in
+branch.<name>.remote and branch.<name>.merge options will be used; see
+linkgit:git-config[1] for details.  If you are currently not on any
+branch or if the current branch does not have a configured upstream,
+the rebase will abort.
+
 All changes made by commits in the current branch but that are not
 in <upstream> are saved to a temporary area.  This is the same set
 of commits that would be shown by `git log <upstream>..HEAD` (or
@@ -216,7 +222,8 @@ leave out at most one of A and B, in which case it defaults to HEAD.
 
 <upstream>::
 	Upstream branch to compare against.  May be any valid commit,
-	not just an existing branch name.
+	not just an existing branch name. Defaults to the configured
+	upstream for the current branch.
 
 <branch>::
 	Working branch; defaults to HEAD.