Check fast-forward more precisely:

The original issue is that (#260), some people want to be able to merge
a fast-forward MR regardless it's rebased or not. The previously
check for this: `target_sha == source_sha_parent`, assumes that the
questioning MR has a linear history (i.e. it's rebased), therefore
we could do a fast-forward merge. However, we could also do fast-forward
merge if the MR has already merged target branch (i.e. it's merged).

This MR would allow them to merge any fast-forward MR regardless rebased
or not.

But actually this breaks some assumption. In the option of:

> Merge commit with semi-linear history
>> 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.
>> When the branch has not been rebased, the user is given the option to do
>> so.

It clearly says: `merging is only allowed if the branch has been rebased`
This MR would break this assumption. The same applies to:

> Fast-forward merge
>> No merge commits are created and all merges are fast-forwarded, which
>> means that merging is only allowed if the branch has been rebased.
>> When the branch has not been rebased, the user is given the option to do
>> so.

This means that rebase and FF are two separated concept and we're
mixing them here. We should probably update the wordings and perhaps
provide another option if we want to keep all the workflows.

My own thought is actually, allowing only FF merge might make little
sense. Allowing only FF *and* rebased (linear history) would make more
sense. That means I think the original behaviour actually makes more
sense, just that the wordings are wrong. i.e. They could be FF merged,
but that's not the point, we still want you to rebase this branch!

On the other hand, I am not against adding a new option for this
behaviour either. People have different workflows, and I think it makes
sense to allow any workflows which Git provided.

Either way, I think some wordings should be changed.

app/models/merge_request.rb

@@ -607,7 +607,9 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def ff_merge_possible?
-    target_sha == source_sha_parent
+    target_project.repository.count_commits_between(
+      target_sha,
+      last_commit.sha) > 0
   end
 
   def must_be_rebased?