Upgrade gitlab_git to version 7.2.6 to fix Error 500 when creating network graphs

Closes #745

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 7.14.0 (unreleased)
+  - Upgrade gitlab_git to version 7.2.6 to fix Error 500 when creating network graphs (Stan Hu)
   - Fix URL used for refreshing notes if relative_url is present (Bartłomiej Święcki)
   - Fix commit data retrieval when branch name has single quotes (Stan Hu)
   - Fix Error 500 when browsing projects with no HEAD (Stan Hu)

Gemfile

@@ -38,7 +38,7 @@ gem "browser", '~> 0.8.0'
 
 # Extracting information from a git repository
 # Provide access to Gitlab::Git library
-gem "gitlab_git", '~> 7.2.5'
+gem "gitlab_git", '~> 7.2.6'
 
 # Ruby/Rack Git Smart-HTTP Server Handler
 # GitLab fork with a lot of changes (improved thread-safety, better memory usage etc)

Gemfile.lock

@@ -271,7 +271,7 @@ GEM
       mime-types (~> 1.19)
     gitlab_emoji (0.1.0)
       gemojione (~> 2.0)
-    gitlab_git (7.2.5)
+    gitlab_git (7.2.6)
       activesupport (~> 4.0)
       charlock_holmes (~> 0.6)
       gitlab-linguist (~> 3.0)
@@ -784,7 +784,7 @@ DEPENDENCIES
   gitlab-grack (~> 2.0.2)
   gitlab-linguist (~> 3.0.1)
   gitlab_emoji (~> 0.1)
-  gitlab_git (~> 7.2.5)
+  gitlab_git (~> 7.2.6)
   gitlab_meta (= 7.0)
   gitlab_omniauth-ldap (= 1.2.1)
   gollum-lib (~> 4.0.2)