mu4e-actions.el: better last dir handling for mu4e-action-git-apply-patch

add-to-list does pre-pend an element to a list by default, but only if
it isn't in there already. This change ensures the path we have just
added is always the first in the list. This makes sense when applying
a series of patches by hand.
mu4e-actions.el: add mu4e-action-git-apply-mbox example

Apply patch is useful when you might want to change the patch. However
for reviews you may well just want stuff applied directly to the
commit history.
Merge pull request #308 from stsquad/more-git-fixes

* add mu4e-action-git-apply-mbox, and improve last-dir handling