emacs: set default in notmuch-read-query

This adds the current query as a "default value" to
notmuch-read-qeury. The default value is available via a down-arrow as
opposed to history which is available from the up arrow.

Note if a user presses return in the minibuffer this value is not
returned.

The implementation is simple but notmuch-read-query could be called
via notmuch-search/notmuch-tree etc from any buffer so it makes sense
to put the decision of how to extract the current query in
notmuch-read-query rather than in each of the callers.
nmbug: Add a git_with_status helper function

Sometimes we want to catch Git errors and handle them, instead of
dying with an error message.  This lower-level version of git() allows
us to get the error status when we want it.
emacs: tree/show remove duplicate function

tree overrides notmuch-show-get-prop so that it can use many of the
utility function directly. Now that tree is in mainline the version
from tree can be moved to show and the original overridden show
version dropped.
Enable Travis-CI as a backup continuous integration service.

You can access the dashboard at https://travis-ci.org/notmuch/notmuch