These two patches fix bugs in and improve on
notmuch-combine-face-text-property.  The bug fixed by the first patch
was found by Damien and the features added in the second patch should
help with notmuch-tagger.

(Sorry for the delay with these.  I've had them sitting in my tree for
a while with the intent to write some nice unit tests for them, but
real life got in the way and I probably won't have time to do anything
nontrivial for a while.)