rebase -i: provide reasonable reflog for the rebased branch

If your rebase succeeded, the HEAD's reflog will still show the whole
mess, but "<branchname>@{1}" now shows the state _before_ the rebase,
so that you can reset (or compare) the original and the rebased
revisions more easily.

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

git-rebase--interactive.sh

@@ -151,8 +151,14 @@ do_next () {
 	esac
 	test -s "$TODO" && return
 
-	HEAD=$(git rev-parse HEAD)
-	HEADNAME=$(cat "$DOTEST"/head-name)
+	comment_for_reflog finish &&
+	HEADNAME=$(cat "$DOTEST"/head-name) &&
+	OLDHEAD=$(cat "$DOTEST"/head) &&
+	SHORTONTO=$(git rev-parse --short $(cat "$DOTEST"/onto)) &&
+	NEWHEAD=$(git rev-parse HEAD) &&
+	message="$GIT_REFLOG_ACTION: $HEADNAME onto $SHORTONTO)" &&
+	git update-ref -m "$message" $HEADNAME $NEWHEAD $OLDHEAD &&
+	git symbolic-ref HEAD $HEADNAME &&
 	rm -rf "$DOTEST" &&
 	warn "Successfully rebased and updated $HEADNAME."
 

t/t3404-rebase-interactive.sh

@@ -99,6 +99,10 @@ test_expect_success 'rebase on top of a non-conflicting commit' '
 	test $(git rev-parse I) = $(git rev-parse HEAD~2)
 '
 
+test_expect_success 'reflog for the branch shows state before rebase' '
+	test $(git rev-parse branch1@{1}) = $(git rev-parse original-branch1)
+'
+
 test_expect_success 'exchange two commits' '
 	FAKE_LINES="2 1" git rebase -i HEAD~2 &&
 	test H = $(git cat-file commit HEAD^ | tail -n 1) &&