rebase: omit patch-identical commits with --fork-point

When the `--fork-point` argument was added to `git rebase`, we changed
the value of $upstream to be the fork point instead of the point from
which we want to rebase.  When $orig_head..$upstream is empty this does
not change the behaviour, but when there are new changes in the upstream
we are no longer checking if any of them are patch-identical with
changes in $upstream..$orig_head.

Fix this by introducing a new variable to hold the fork point and using
this to restrict the range as an extra (negative) revision argument so
that the set of desired revisions becomes (in fork-point mode):

	git rev-list --cherry-pick --right-only \
		$upstream...$orig_head ^$fork_point

This allows us to correctly handle the scenario where we have the
following topology:

	    C --- D --- E  <- dev
	   /
	  B  <- master@{1}
	 /
	o --- B' --- C* --- D*  <- master

where:
- B' is a fixed-up version of B that is not patch-identical with B;
- C* and D* are patch-identical to C and D respectively and conflict
  textually if applied in the wrong order;
- E depends textually on D.

The correct result of `git rebase master dev` is that B is identified as
the fork-point of dev and master, so that C, D, E are the commits that
need to be replayed onto master; but C and D are patch-identical with C*
and D* and so can be dropped, so that the end result is:

	o --- B' --- C* --- D* --- E  <- dev

If the fork-point is not identified, then picking B onto a branch
containing B' results in a conflict and if the patch-identical commits
are not correctly identified then picking C onto a branch containing D
(or equivalently D*) results in a conflict.

This change allows us to handle both of these cases, where previously we
either identified the fork-point (with `--fork-point`) but not the
patch-identical commits *or* (with `--no-fork-point`) identified the
patch-identical commits but not the fact that master had been rewritten.

Reported-by: Ted Felix <ted@tedfelix.com>
Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 80e0a951e..47b568266 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -169,6 +169,29 @@ test_expect_success 'default to common base in @{upstream}s reflog if no upstrea
 	test_cmp expect actual
 '
 
+test_expect_success 'cherry-picked commits and fork-point work together' '
+	git checkout default-base &&
+	echo Amended >A &&
+	git commit -a --no-edit --amend &&
+	test_commit B B &&
+	test_commit new_B B "New B" &&
+	test_commit C C &&
+	git checkout default &&
+	git reset --hard default-base@{4} &&
+	test_commit D D &&
+	git cherry-pick -2 default-base^ &&
+	test_commit final_B B "Final B" &&
+	git rebase &&
+	echo Amended >expect &&
+	test_cmp A expect &&
+	echo "Final B" >expect &&
+	test_cmp B expect &&
+	echo C >expect &&
+	test_cmp C expect &&
+	echo D >expect &&
+	test_cmp D expect
+'
+
 test_expect_success 'rebase -q is quiet' '
 	git checkout -b quiet topic &&
 	git rebase -q master >output.out 2>&1 &&