[RFC WIP PATCH v8 10/13] rebase: don't rebase linear topology with --fork-point

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Fix a regression introduced in 4f21454b55 ("merge-base: handle
--fork-point without reflog", 2016-10-12).

Before that change having a linear history on top of an upstream
master would with --fork-point (aka argument-less rebase) tell us
there was nothing to be done:

    $ git rebase
    Current branch master is up to date.

After that change "rebase" will always redundantly find that it has
work to do (it doesn't):

    $ git rebase
    First, rewinding head to replay your work on top of it...
    Applying: [...]

Whereas equivalently running:

    $ git rebase @{upstream}
    $ git rebase $(git merge-base --fork-point @{u})

Gives us the old behavior of doing nothing.

Now, why did we have this regression? Fully digging into it yields an
interesting combination of causes:

Way back in 1308c17b3e ("Allow rebase to run if upstream is completely
merged", 2007-07-04) "rebase" learned to not do this redundant work
when asked to rebase on a commit that was already an ancestor of the
current commit.

Then in 1e0dacdbdb ("rebase: omit patch-identical commits with
--fork-point", 2014-07-16) a rebase bug was fixed for a case where the
history to be rebased was divergent by entirely skipping the 2007-era
logic if --fork-point was provided.

But here's the critical thing, *only* if the --fork-point was
divergent. At that time "git merge-base --fork-point A B" would return
nothing if the two commits weren't divergent.

Then in 4f21454b55 ("merge-base: handle --fork-point without reflog",
2016-10-12) which introduced the regression being fixed here, a bug
fix for "git merge-base --fork-point" being run stand-alone by proxy
broke this use-case git-rebase.sh was relying on, since it was still
assuming that if we didn't have divergent history we'd have no output.

Finally, when "rebase" was rewritten in C a combination of
9a48a615b4 ("builtin rebase: try to fast forward when possible",
2018-09-04), 103148aad8 ("merge-base --fork-point: extract libified
function", 2018-09-04) and 92d0d74e8d ("builtin rebase: support
`fork-point` option", 2018-09-04) faithfully re-implemented the
then-buggy behavior.

So let's fix this. It's easy enough, we just stop explicitly excluding
--fork-point from the can_fast_forward(...) test we're doing, which as
discussed above is faithfully ported over from buggy shellscript-era
logic.

I'm not bothering to fix this in the legacy rebase mode. As discussed
in 9aea5e9286 ("rebase: fix regression in rebase.useBuiltin=false test
mode", 2019-02-13) it'll be going away shortly after 2.21.0 lands.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 t/t3421-rebase-topology-linear.sh | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/t/t3421-rebase-topology-linear.sh b/t/t3421-rebase-topology-linear.sh
index b847064f91..1754537789 100755
--- a/t/t3421-rebase-topology-linear.sh
+++ b/t/t3421-rebase-topology-linear.sh
@@ -55,6 +55,21 @@ test_run_rebase success -m
 test_run_rebase success -i
 test_have_prereq !REBASE_P || test_run_rebase success -p
 
+test_run_rebase () {
+	result=$1
+	shift
+	test_expect_$result "rebase $* is no-op if remote upstream is an ancestor" "
+		reset_rebase &&
+		GIT_TEST_REBASE_USE_BUILTIN=true git rebase $* branch-b branch-e &&
+		test_cmp_rev e HEAD
+	"
+}
+test_run_rebase success ''
+test_run_rebase success --fork-point
+test_run_rebase success -m
+test_run_rebase success -i
+test_have_prereq !REBASE_P || test_run_rebase success -p
+
 test_run_rebase () {
 	result=$1
 	shift
-- 
2.21.0.1020.gf2820cf01a