Added --changed option to "brew tests" dev cmd

This option allows the user to run tests on all
files that have been changed from the master
branch.

Library/Homebrew/dev-cmd/tests.rb

@@ -26,6 +26,8 @@ def tests_args
                           "official external commands."
       switch "--byebug",
              description: "Enable debugging using byebug."
+      switch "--changed",
+             description: "Only runs tests on files that were changed from the master branch."
       flag   "--only=",
              description: "Run only <test_script>`_spec.rb`. Appending `:`<line_number> will start at a "\
                           "specific line."
@@ -124,10 +126,28 @@ def tests
           parallel = false
           ["test/#{test_name}_spec.rb:#{line}"]
         end
+      elsif args.changed?
+        changed_files = Utils.popen_read("git", "diff", "--name-only", "master")
+
+        filestub_regex = %r{Library/Homebrew/([A-Za-z/_-]+).rb}
+
+        changed_files.scan(filestub_regex)
+                     .map { |filestub| "test/#{filestub.last}_spec.rb" }
+                     .select { |filename| File.exist?(filename) }
       else
         Dir.glob("test/**/*_spec.rb")
       end
 
+      if files.empty?
+        raise UsageError, "The --only= argument requires a valid file or folder name!" if args.only
+
+        if args.changed?
+          opoo "Either no files have been changed from the master branch " \
+               "or no tests are associated with the changed files!"
+          return
+        end
+      end
+
       parallel_rspec_log_name = "parallel_runtime_rspec"
       parallel_rspec_log_name = "#{parallel_rspec_log_name}.no_compat" if args.no_compat?
       parallel_rspec_log_name = "#{parallel_rspec_log_name}.generic" if args.generic?