SVN commit 12927 by rakuco: qt5-core: Update patch-src_corelib_thread_qmutex_cpp.patch and add context.
Update the patch to the latest version of the email thread I'm CC'ed to with Gleb Popov, Thiago Macieira and Olivier Goffart. While here, briefly describe what this is supposed to fix while a patch with a proper explanation does not make its way to qtbase upstream. M +16 -7 patch-src_corelib_thread_qmutex_cpp.patch
