[danger] Don't check changelogs on non-source changes (#4115)

* Only check changelogs on PRs that modify objc/objc++ source

* Fix trailing whitespace

* Fix style and add swift files to changelog check

* Fix trailing whitespace

Author: morganchen12

Dangerfile

@@ -4,7 +4,8 @@
 # Taken from samdmarshall/danger
 def didModify(files_array)
   files_array.each do |file_name|
-    if git.modified_files.include?(file_name) || git.deleted_files.include?(file_name)
+    if git.modified_files.include?(file_name) ||
+       git.deleted_files.include?(file_name)
       return true
     end
   end
@@ -93,11 +94,30 @@ has_license_changes = didModify(["LICENSE"])
 @has_googleutilities_changes = hasChangesIn("GoogleUtilities/")
 @has_zipbuilder_changes = hasChangesIn("ZipBuilder/")
 
+# A FileList containing ObjC, ObjC++ or C++ changes.
+sdk_changes = (git.modified_files +
+               git.added_files +
+               git.deleted_files).select do |line|
+  line.end_with?(".h") ||
+    line.end_with?(".m") ||
+    line.end_with?(".mm") ||
+    line.end_with?(".cc") ||
+    line.end_with?(".swift")
+end
+
+# Whether or not the PR has modified SDK source files.
+has_sdk_changes = sdk_changes.empty?
+
 ### Actions
 
-# Warn if a changelog is left out on a non-trivial PR
-if !has_changelog_changes && !declared_trivial
-  warn("Did you forget to add a changelog entry? (Add `#no-changelog` to the PR description to silence this warning.)")
+# Warn if a changelog is left out on a non-trivial PR that has modified
+# SDK source files (podspec, markdown, etc changes are excluded).
+if has_sdk_changes
+  if !has_changelog_changes && !declared_trivial
+    warning = "Did you forget to add a changelog entry? (Add #no-changelog"\
+      " to the PR description to silence this warning.)"
+    warn(warning)
+  end
 end
 
 # Error on license edits