update rugged

the rugged versions up to 0.26.0b3 had a bug concerning the signature
extraction. The extracted signature was not always the same, probably
due to a buffer (overflow) issue in libgit.

see https://github.com/libgit2/rugged/issues/608

Gemfile

@@ -15,7 +15,7 @@ gem 'default_value_for', '~> 3.0.0'
 gem 'mysql2', '~> 0.4.5', group: :mysql
 gem 'pg', '~> 0.18.2', group: :postgres
 
-gem 'rugged', '~> 0.25.1.1'
+gem 'rugged', '~> 0.26.0'
 gem 'grape-route-helpers', '~> 2.0.0'
 
 gem 'faraday', '~> 0.12'

Gemfile.lock

@@ -751,7 +751,7 @@ GEM
     rubyzip (1.2.1)
     rufus-scheduler (3.4.0)
       et-orbi (~> 1.0)
-    rugged (0.25.1.1)
+    rugged (0.26.0)
     safe_yaml (1.0.4)
     sanitize (2.1.0)
       nokogiri (>= 1.4.4)
@@ -1084,7 +1084,7 @@ DEPENDENCIES
   ruby-prof (~> 0.16.2)
   ruby_parser (~> 3.8)
   rufus-scheduler (~> 3.4)
-  rugged (~> 0.25.1.1)
+  rugged (~> 0.26.0)
   sanitize (~> 2.0)
   sass-rails (~> 5.0.6)
   scss_lint (~> 0.54.0)

lib/gitlab/git/commit.rb

@@ -336,7 +336,7 @@ module Gitlab
         begin
           raw_commit.to_mbox(options)
         rescue Rugged::InvalidError => ex
-          if ex.message =~ /Commit \w+ is a merge commit/
+          if ex.message =~ /commit \w+ is a merge commit/i
             'Patch format is not currently supported for merge commits.'
           end
         end