Update wordings accordingly because:

It's not that it must be rebased, it could also be merged.
This also raises a question: Why we could only rebase on
GitLab, but not merge whenever merge should also be an option?

After this change, we could also not enforce a linear history
because we also allow merging.

app/models/merge_request.rb

@@ -268,7 +268,7 @@ class MergeRequest < ActiveRecord::Base
 
     check_if_can_be_merged
 
-    can_be_merged? && !must_be_rebased?
+    can_be_merged? && !should_be_rebased?
   end
 
   def gitlab_merge_status
@@ -612,7 +612,7 @@ class MergeRequest < ActiveRecord::Base
       last_commit.sha) > 0
   end
 
-  def must_be_rebased?
+  def should_be_rebased?
     self.project.ff_merge_must_be_possible? && !ff_merge_possible?
   end
 

app/views/projects/_merge_request_settings.html.haml

@@ -19,11 +19,11 @@
         %strong Merge commit with semi-linear history
         %br
         %span.descr
-          A merge commit is created for every merge, but merging is only allowed if the branch has been rebased.
-          This way you get a history that reads linearly (as with fast-forward merges), with the addition of merge commits.
+          A merge commit is created for every merge, but merging is only allowed if fast-forward merge is possible.
+          This way you could make sure that if this merge request would build, after merging to target branch it would also build.
         %br
         %span.descr
-          When the branch has not been rebased, the user is given the option to do so.
+          When fast-forward merge is not possible, the user is given the option to rebase.
 
     .radio
       = label_tag :project_merge_method_ff do
@@ -31,10 +31,10 @@
         %strong Fast-forward merge
         %br
         %span.descr
-          No merge commits are created and all merges are fast-forwarded, which means that merging is only allowed if the branch has been rebased.
+          No merge commits are created and all merges are fast-forwarded, which means that merging is only allowed if the branch could be fast-forwarded.
         %br
         %span.descr
-          When the branch has not been rebased, the user is given the option to do so.
+          When fast-forward merge is not possible, the user is given the option to rebase.
 
   .form-group
     = f.label :merge_requests_template, class: 'label-light' do

app/views/projects/merge_requests/widget/_open.html.haml

@@ -21,7 +21,7 @@
       = render 'projects/merge_requests/widget/open/merge_when_build_succeeds'
     - elsif !@merge_request.can_be_merged_by?(current_user)
       = render 'projects/merge_requests/widget/open/not_allowed'
-    - elsif @merge_request.must_be_rebased?
+    - elsif @merge_request.should_be_rebased?
       = render 'projects/merge_requests/widget/open/rebase'
     - else
       = render 'projects/merge_requests/widget/open/accept'