pull --rebase: be cleverer with rebased upstream branches

When the upstream branch is tracked, we can detect if that branch
was rebased since it was last fetched.  Teach git to use that
information to rebase from the old remote head onto the new remote head.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -106,6 +106,15 @@ error_on_no_merge_candidates () {
 	exit 1
 }
 
+test true = "$rebase" && {
+	. git-parse-remote &&
+	origin="$1"
+	test -z "$origin" && origin=$(get_default_remote)
+	reflist="$(get_remote_refs_for_fetch "$@" 2>/dev/null |
+		sed "s|refs/heads/\(.*\):|\1|")" &&
+	oldremoteref="$(git rev-parse --verify \
+		"refs/remotes/$origin/$reflist" 2>/dev/null)"
+}
 orig_head=$(git rev-parse --verify HEAD 2>/dev/null)
 git-fetch --update-head-ok "$@" || exit 1
 
@@ -164,6 +173,7 @@ then
 fi
 
 merge_name=$(git fmt-merge-msg <"$GIT_DIR/FETCH_HEAD") || exit
-test true = "$rebase" && exec git-rebase $merge_head
+test true = "$rebase" &&
+	exec git-rebase --onto $merge_head ${oldremoteref:-$merge_head}
 exec git-merge $no_summary $no_commit $squash $no_ff $strategy_args \
 	"$merge_name" HEAD $merge_head