[PATCH] rebase: use reflog to find common base with upstream

[John Keeping <john@keeping.me.uk>]

Commit 15a147e (rebase: use @{upstream} if no upstream specified,
2011-02-09) says:

	Make it default to 'git rebase @{upstream}'. That is also what
	'git pull [--rebase]' defaults to, so it only makes sense that
	'git rebase' defaults to the same thing.

but that isn't actually the case.  Since commit d44e712 (pull: support
rebased upstream + fetch + pull --rebase, 2009-07-19), pull has actually
chosen the most recent reflog entry which is an ancestor of the current
branch if it can find one.

Change rebase so that it uses the same logic.

Signed-off-by: John Keeping <john@keeping.me.uk>
---
 git-rebase.sh     | 8 ++++++++
 t/t3400-rebase.sh | 6 ++++--
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/git-rebase.sh b/git-rebase.sh
index 226752f..fd36cf7 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -437,6 +437,14 @@ then
 			error_on_missing_default_upstream "rebase" "rebase" \
 				"against" "git rebase <branch>"
 		fi
+		for reflog in $(git rev-list -g "$upstream_name" 2>/dev/null)
+		do
+			if test "$reflog" = "$(git merge-base "$reflog" HEAD)"
+			then
+				upstream_name=$reflog
+				break
+			fi
+		done
 		;;
 	*)	upstream_name="$1"
 		shift
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index ebf93b0..4f45f7e 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -134,12 +134,14 @@ test_expect_success 'fail when upstream arg is missing and not configured' '
 	test_must_fail git rebase
 '
 
-test_expect_success 'default to @{upstream} when upstream arg is missing' '
+test_expect_success 'default to common base in @{upstream}s reflog if no upstream arg' '
 	git checkout -b default topic &&
 	git config branch.default.remote . &&
 	git config branch.default.merge refs/heads/master &&
 	git rebase &&
-	test "$(git rev-parse default~1)" = "$(git rev-parse master)"
+	git merge-base topic master >expect &&
+	git rev-parse default~1 >actual &&
+	test_cmp expect actual
 '
 
 test_expect_success 'rebase -q is quiet' '
-- 
1.8.4.566.g73d370b