detect pull requests that have been applied to the current branch.
add magit-gh-pulls-mode-lighter
remove unnecessary progn forms
require pcache
enforce use of spaces for indentation
magit-insert-gh-pulls: fontify header like others

Fontify only the text of the header not the hole line;
like it is done for other section headers.
don't use magit-define-inserter

as it has been removed from Magit.
don't use magit-git-log-options

as it has been removed from Magit.