Fixed #11240 -- Made makemessages i18n command escape % symbols in literals...

Fixed #11240 -- Made makemessages i18n command escape % symbols in literals passed to the trans tag.

This avoids problems with unintended automatic detection, marking and
validation of Python string formatting specifiers performed by
xgettext(1)/msgfmt(1) that stem from the fact that under the hood makemessages
converts templates to Python code before passing them to xgettext.

This also makes it consistent with its behavior on literals passed to the
blocktrans tag.

Thanks Jannis and claude for reviewing and feedback.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17190 bcc190cf-cafb-0310-a4f2-bffc1f526a37