Create helper to get all changed files

Danger apparently has three different objects which could contain files
you often want to check:

 - git.added_files
 - git.modified_files
 - git.renamed_files

The problem: If a file is renamed, `modified_files` contains the file
path before the rename. In some Danger checks we use `added_files` +
`modified_files`, which might contain the deleted paths of renamed
files, but missing the new paths of renamed files.

So we need to consider `renamed_files` as well.

1 files changed, 34 insertions, 0 deletions

diff --git a/danger/plugins/helper.rb b/danger/plugins/helper.rb
new file mode 100644
index 00000000000..f4eb9119266
--- /dev/null
+++ b/danger/plugins/helper.rb
@@ -0,0 +1,34 @@
+# frozen_string_literal: true
+
+module Danger
+  # Common helper functions for our danger scripts
+  # If we find ourselves repeating code in our danger files, we might as well put them in here.
+  class Helper < Plugin
+    # Returns a list of all files that have been added, modified or renamed.
+    # `git.modified_files` might contain paths that already have been renamed,
+    # so we need to remove them from the list.
+    #
+    # Considering these changes:
+    #
+    # - A new_file.rb
+    # - D deleted_file.rb
+    # - M modified_file.rb
+    # - R renamed_file_before.rb -> renamed_file_after.rb
+    #
+    # it will return
+    # ```
+    # [ 'new_file.rb', 'modified_file.rb', 'renamed_file_after.rb' ]
+    # ```
+    #
+    # @return [Array<String>]
+    def all_changed_files
+      Set.new
+        .merge(git.added_files.to_a)
+        .merge(git.modified_files.to_a)
+        .merge(git.renamed_files.map { |x| x[:after] })
+        .subtract(git.renamed_files.map { |x| x[:before] })
+        .to_a
+        .sort
+    end
+  end
+end