Merge branch 'klowner/gitlab-ce-fix-compare-by-tag-crash' into 'master'

Fix 500 error when comparing by tags

_Originally opened at !2787 by @klowner._

- - -

Fixes #13171.
Fixes #13370.

See merge request !2849

app/models/repository.rb

@@ -611,6 +611,8 @@ class Repository
   end
 
   def merge_base(first_commit_id, second_commit_id)
+    first_commit_id = commit(first_commit_id).try(:id) || first_commit_id
+    second_commit_id = commit(second_commit_id).try(:id) || second_commit_id
     rugged.merge_base(first_commit_id, second_commit_id)
   rescue Rugged::ReferenceError
     nil