Allow rebase to run if upstream is completely merged

Consider this history:

  o--o-...-B          <- origin
      \     \
       x--x--M--x--x  <- master

In this situation, rebase considers master fully up-to-date and would
not do anything. However, if there were additional commits on origin,
the rebase would run and move the commits x on top of origin.

Here we change rebase to short-circuit out only if the history since origin
is strictly linear. Consequently, the above as well as a history like this
would be linearized:

  o--o               <- origin
      \
       x--x
        \  \
         x--M--x--x  <- master

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3400-rebase.sh

@@ -12,7 +12,7 @@ This test runs git rebase and checks that the author information is not lost.
 export GIT_AUTHOR_EMAIL=bogus_email_address
 
 test_expect_success \
-    'prepare repository with topic branch, then rebase against master' \
+    'prepare repository with topic branches' \
     'echo First > A &&
      git update-index --add A &&
      git-commit -m "Add A." &&
@@ -24,11 +24,48 @@ test_expect_success \
      echo Third >> A &&
      git update-index A &&
      git-commit -m "Modify A." &&
+     git checkout -b side my-topic-branch &&
+     echo Side >> C &&
+     git add C &&
+     git commit -m "Add C" &&
+     git checkout -b nonlinear my-topic-branch &&
+     echo Edit >> B &&
+     git add B &&
+     git commit -m "Modify B" &&
+     git merge side &&
+     git checkout -b upstream-merged-nonlinear &&
+     git merge master &&
      git checkout -f my-topic-branch &&
+     git tag topic
+'
+
+test_expect_success 'rebase against master' '
      git rebase master'
 
 test_expect_failure \
     'the rebase operation should not have destroyed author information' \
     'git log | grep "Author:" | grep "<>"'
 
+test_expect_success 'rebase after merge master' '
+     git reset --hard topic &&
+     git merge master &&
+     git rebase master &&
+     ! git show | grep "^Merge:"
+'
+
+test_expect_success 'rebase of history with merges is linearized' '
+     git checkout nonlinear &&
+     test 4 = $(git rev-list master.. | wc -l) &&
+     git rebase master &&
+     test 3 = $(git rev-list master.. | wc -l)
+'
+
+test_expect_success \
+    'rebase of history with merges after upstream merge is linearized' '
+     git checkout upstream-merged-nonlinear &&
+     test 5 = $(git rev-list master.. | wc -l) &&
+     git rebase master &&
+     test 3 = $(git rev-list master.. | wc -l)
+'
+
 test_done