Rebase CE branch onto CE/EE merge base before diff

If CE master is ahead of EE master, and the CE branch
was forked from CE master, then the CE branch would
contain a lot of changes which aren't presented in EE master,
therefore they're not presented in EE branch, either.

In this case, we need to remove the commits from CE master
which aren't merged into EE yet. So we want to rebase CE branch
onto CE/EE merge base, with ce_fetch_base and ce_fetch_head,
where ce_fetch_base is the merge base of CE master and CE branch,
and ce_fetch_head is the original HEAD of CE branch.

This should make sure there's no extra commits involved.

The only way it could fail would be conflicts during rebase,
which ideally should not happen at all. This doesn't mean
it's impossible to happen, but given the reason we make the CE branch,
it should only contain minimum changes from EE to CE, which shouldn't
conflict.

Let's try this and see and investigate if conflicts happen.

scripts/ee_specific_check/ee_specific_check.rb

+# frozen_string_literal: true
+
 module EESpecificCheck
   WHITELIST = [
     'CHANGELOG-EE.md',
@@ -21,15 +23,15 @@ module EESpecificCheck
   def find_compare_base
     setup_canonical_remotes
 
-    fetch_head_ce = fetch_remote_ce_branch
-    ce_merge_base = run_git_command("merge-base #{fetch_head_ce} HEAD").strip
-    ee_merge_base = run_git_command("merge-base canonical-ee/master HEAD").strip
+    ce_fetch_head = fetch_remote_ce_branch
+    ce_merge_base = run_git_command("merge-base canonical-ce/master canonical-ee/master")
+    ee_merge_base = run_git_command("merge-base canonical-ee/master HEAD")
 
     ce_updated_base =
-      if fetch_head_ce.start_with?('canonical-ce')
+      if ce_fetch_head.start_with?('canonical-ce')
         ce_merge_base # Compare with merge-base if no specific CE branch
       else
-        fetch_head_ce # Compare with the new HEAD if we do have one
+        checkout_and_rebase_ce_fetch_head_onto_ce_merge_base(ce_merge_base, ce_fetch_head)
       end
 
     CompareBase.new(ce_merge_base, ee_merge_base, ce_updated_base)
@@ -38,9 +40,9 @@ module EESpecificCheck
   def setup_canonical_remotes
     run_git_command(
       "remote add canonical-ee https://gitlab.com/gitlab-org/gitlab-ee.git",
-      "remote add canonical-ce https://gitlab.com/gitlab-org/gitlab-ce.git")
-
-    run_git_command("fetch canonical-ee master --quiet")
+      "remote add canonical-ce https://gitlab.com/gitlab-org/gitlab-ce.git",
+      "fetch canonical-ee master --quiet",
+      "fetch canonical-ce master --quiet")
   end
 
   def fetch_remote_ce_branch
@@ -51,6 +53,77 @@ module EESpecificCheck
     "#{remote_to_fetch}/#{branch_to_fetch}"
   end
 
+  def checkout_and_rebase_ce_fetch_head_onto_ce_merge_base(ce_merge_base, ce_fetch_head)
+    # So that we could switch back
+    head = head_commit_sha
+
+    # Use detached HEAD so that we don't update HEAD
+    run_git_command("checkout #{ce_fetch_head}")
+
+    # We rebase onto the commit which is the latest commit presented in both
+    # CE and EE, i.e. ce_merge_base, cutting off commits aren't merged into
+    # EE yet. Here's an example:
+    #
+    # * o: Relevant commits
+    # * x: Irrelevant commits
+    # * !: Commits we want to cut off from CE branch
+    #
+    #                ^-> o CE branch (ce_fetch_head)
+    #               /
+    #     o -> o -> ! -> x CE master
+    #          v (ce_merge_base)
+    #     o -> o -> o -> x EE master
+    #               \ (ee_merge_base)
+    #                v-> o EE branch
+    #
+    # We want to rebase above into this: (we only change the connection)
+    #
+    #            -> - -> o CE branch (ce_fetch_head)
+    #           /
+    #     o -> o -> ! -> x CE master
+    #          v (ce_merge_base)
+    #     o -> o -> o -> x EE master
+    #               \ (ee_merge_base)
+    #                v-> o EE branch
+    #
+    # Therefore we rebase onto ce_merge_base, which is based off CE master,
+    # for the CE branch (ce_fetch_head), effective remove the commit marked
+    # as ! in the graph for CE branch. We need to remove it because it's not
+    # merged into EE yet, therefore won't be available in the EE branch.
+    #
+    # After rebase is done, then we could compare against
+    # ce_merge_base..ee_merge_base along with ce_fetch_head..HEAD (EE branch)
+    # where ce_merge_base..ee_merge_base is the update-to-date
+    # CE/EE difference and ce_fetch_head..HEAD is the changes we made in
+    # CE and EE branches.
+    run_git_command("rebase --onto #{ce_merge_base} canonical-ce/master #{ce_fetch_head}")
+
+    if status_clean?
+      head_commit_sha
+    else
+      say <<~MESSAGE
+        💥 WE HAVE CONFLICTS! This shouldn't happen. Please create an issue
+        💥 and ping @godfat to investigate.
+      MESSAGE
+
+      run_git_command("rebase --abort")
+
+      exit(255)
+    end
+
+  ensure # ensure would still run if we call exit, don't worry
+    # Make sure to switch back
+    run_git_command("checkout #{head}")
+  end
+
+  def head_commit_sha
+    run_git_command("rev-parse HEAD")
+  end
+
+  def status_clean?
+    run_git_command("status --porcelain") == ''
+  end
+
   def remove_remotes
     run_git_command(
       "remote remove canonical-ee",
@@ -119,7 +192,7 @@ module EESpecificCheck
     commands.map do |cmd|
       puts "=> Running `#{cmd}`"
 
-      `#{cmd}`
+      `#{cmd}`.strip
     end
   end
 end