(mutually_exclusive_p): Fix the regression from commit 6fad73d7cc53

(mutually_exclusive_p): Fix the regression from commit 6fad73d7cc53

Commit 6fad73d7cc53 throws away some useful optimization because
it misfired in some cases (as seen in bug#657260).  Here we try to
recover those useful optimizations with a slightly more careful
algorithm.

* src/regex-emacs.c (mutually_exclusive_aux): Rename from
`mutually_exclusive_p`.  Add two new args.  Improve the
case where we need to recurse.
(mutually_exclusive_p): New function defined on top of it.

* test/src/regex-emacs-tests.el (regexp-tests-backtrack-optimization):
Re-enable the test.