+branch=$(git-rev-parse --verify "${branch_name}^0") || exit
+
+# Make sure the branch to rebase onto is valid.
+onto_name=${newbase-"$upstream_name"}
+onto=$(git-rev-parse --verify "${onto_name}^0") || exit
+
+# Now we are rebasing commits $upstream..$branch on top of $onto
+
+# 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"
+then
+ mb=$(git-merge-base "$onto" "$branch")
+ if test "$mb" = "$onto"
+ then
+ echo >&2 "Current branch $branch_name is up to date."
+ exit 0
+ fi
+fi
+
+# Rewind the head to "$onto"; this saves our current head in ORIG_HEAD.
+git-reset --hard "$onto"