Merge branch 'relative_links_default_branch' into 'master'

Relative links in project README should point to default branch, not master

Relative links in the README shown on a project's main page currently always point to the "master" branch.  This is a particularly obvious problem for projects that have no "master" branch, since the link points to a non-existent file.

This patch changes the relative links to instead point to the default branch.

Resolves #2707, #1134, #1852

See merge request !1801

CHANGELOG

@@ -39,6 +39,7 @@ v 8.2.0 (unreleased)
   - Improve Continuous Integration graphs page
   - Make color of "Accept Merge Request" button consistent with current build status
   - Add ignore white space option in merge request diff and commit and compare view
+  - Relative links from a repositories README.md now link to the default branch
 
 v 8.1.4
   - Fix bug where manually merged branches in a MR would end up with an empty diff (Stan Hu)

lib/gitlab/markdown/relative_link_filter.rb

@@ -51,7 +51,7 @@ module Gitlab
           relative_url_root,
           context[:project].path_with_namespace,
           path_type(file_path),
-          ref || 'master',  # assume that if no ref exists we can point to master
+          ref || context[:project].default_branch,  # if no ref exists, point to the default branch
           file_path
         ].compact.join('/').squeeze('/').chomp('/')