rebase: Fix the detection of fast-forwarding of the current branch to upstream.

Previously, a rebasing operation with on a branch that is just tracking
an upstream branch would output a confusing "Nothing to do" due to no
patches being given to git-am.

The test brings the behaviour back into line with that of just before
e646c9c8c0.

Signed-off-by: Robert Shearman <rob@codeweavers.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-rebase.sh

@@ -266,14 +266,11 @@ onto=$(git-rev-parse --verify "${onto_name}^0") || exit
 
 # Check if we are already based on $onto, but this should be
 # done only when upstream and onto are the same.
-if test "$upstream" = "$onto"
+mb=$(git-merge-base "$onto" "$branch")
+if test "$upstream" = "$onto" && test "$mb" = "$onto"
 then
-	mb=$(git-merge-base "$onto" "$branch")
-	if test "$mb" = "$onto"
-	then
-		echo >&2 "Current branch $branch_name is up to date."
-		exit 0
-	fi
+	echo >&2 "Current branch $branch_name is up to date."
+	exit 0
 fi
 
 # Rewind the head to "$onto"; this saves our current head in ORIG_HEAD.
@@ -281,7 +278,7 @@ git-reset --hard "$onto"
 
 # If the $onto is a proper descendant of the tip of the branch, then
 # we just fast forwarded.
-if test "$mb" = "$onto"
+if test "$mb" = "$branch"
 then
 	echo >&2 "Fast-forwarded $branch to $newbase."
 	exit 0