Upgrade gitlab_git to 7.2.15 to fix `git blame` errors with ISO-encoded files

Closes https://github.com/gitlabhq/gitlabhq/issues/9577

CHANGELOG

 Please view this file on the master branch, on stable branches it's out of date.
 
 v 8.0.0 (unreleased)
+  - Upgrade gitlab_git to 7.2.15 to fix `git blame` errors with ISO-encoded files (Stan Hu)
   - Prevent too many redirects upon login when home page URL is set to external_url (Stan Hu)
   - Improve dropdown positioning on the project home page (Hannes Rosenögger)
   - Upgrade browser gem to 1.0.0 to avoid warning in IE11 compatibilty mode (Stan Hu)

Gemfile

@@ -38,7 +38,7 @@ gem "browser", '~> 1.0.0'
 
 # Extracting information from a git repository
 # Provide access to Gitlab::Git library
-gem "gitlab_git", '~> 7.2.14'
+gem "gitlab_git", '~> 7.2.15'
 
 # Ruby/Rack Git Smart-HTTP Server Handler
 # GitLab fork with a lot of changes (improved thread-safety, better memory usage etc)

Gemfile.lock

@@ -276,7 +276,7 @@ GEM
       mime-types (~> 1.19)
     gitlab_emoji (0.1.0)
       gemojione (~> 2.0)
-    gitlab_git (7.2.14)
+    gitlab_git (7.2.15)
       activesupport (~> 4.0)
       charlock_holmes (~> 0.6)
       gitlab-linguist (~> 3.0)
@@ -790,7 +790,7 @@ DEPENDENCIES
   gitlab-grack (~> 2.0.2)
   gitlab-linguist (~> 3.0.1)
   gitlab_emoji (~> 0.1)
-  gitlab_git (~> 7.2.14)
+  gitlab_git (~> 7.2.15)
   gitlab_meta (= 7.0)
   gitlab_omniauth-ldap (= 1.2.1)
   gollum-lib (~> 4.0.2)