Use new gitlab_git gem. It makes CONTRIBUTING.md work

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Gemfile

@@ -32,7 +32,7 @@ gem 'omniauth-github'
 
 # Extracting information from a git repository
 # Provide access to Gitlab::Git library
-gem "gitlab_git", '~> 5.5.0'
+gem "gitlab_git", '~> 5.6.0'
 
 # Ruby/Rack Git Smart-HTTP Server Handler
 gem 'gitlab-grack', '~> 2.0.0.pre', require: 'grack'

Gemfile.lock

@@ -181,7 +181,7 @@ GEM
       charlock_holmes (~> 0.6.6)
       escape_utils (~> 0.2.4)
       mime-types (~> 1.19)
-    gitlab_git (5.5.0)
+    gitlab_git (5.6.0)
       activesupport (~> 4.0.0)
       charlock_holmes (~> 0.6.9)
       gitlab-grit (~> 2.6.1)
@@ -590,7 +590,7 @@ DEPENDENCIES
   gitlab-gollum-lib (~> 1.1.0)
   gitlab-grack (~> 2.0.0.pre)
   gitlab-linguist (~> 3.0.0)
-  gitlab_git (~> 5.5.0)
+  gitlab_git (~> 5.6.0)
   gitlab_meta (= 6.0)
   gitlab_omniauth-ldap (= 1.0.4)
   gon (~> 5.0.0)

app/models/tree.rb

@@ -11,7 +11,7 @@ class Tree
       @readme = Gitlab::Git::Blob.find(git_repo, sha, readme_path)
     end
 
-    if contribution_tree = @entries.find(&:contribution?)
+    if contribution_tree = @entries.find(&:contributing?)
       contribution_path = path == '/' ? contribution_tree.name : File.join(path, contribution_tree.name)
       @contribution_guide = Gitlab::Git::Blob.find(git_repo, sha, contribution_path)
     end