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>

Documentation/git-pull.txt

@@ -35,7 +35,11 @@ include::urls-remotes.txt[]
 include::merge-strategies.txt[]
 
 \--rebase::
-	Instead of a merge, perform a rebase after fetching.
+	Instead of a merge, perform a rebase after fetching.  If
+	there is a remote ref for the upstream branch, and this branch
+	was rebased since last fetched, the rebase uses that information
+	to avoid rebasing non-local changes.
+
 	*NOTE:* This is a potentially _dangerous_ mode of operation.
 	It rewrites history, which does not bode well when you
 	published that history already.  Do *not* use this option

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

t/t5520-pull.sh

@@ -71,8 +71,25 @@ test_expect_success 'branch.to-rebase.rebase' '
 	git reset --hard before-rebase &&
 	git config branch.to-rebase.rebase 1 &&
 	git pull . copy &&
+	git config branch.to-rebase.rebase 0 &&
 	test $(git rev-parse HEAD^) = $(git rev-parse copy) &&
 	test new = $(git show HEAD:file2)
 '
 
+test_expect_success '--rebase with rebased upstream' '
+
+	git remote add -f me . &&
+	git checkout copy &&
+	git reset --hard HEAD^ &&
+	echo conflicting modification > file &&
+	git commit -m conflict file &&
+	git checkout to-rebase &&
+	echo file > file2 &&
+	git commit -m to-rebase file2 &&
+	git pull --rebase me copy &&
+	test "conflicting modification" = "$(cat file)" &&
+	test file = $(cat file2)
+
+'
+
 test_done