emacs: show: use interactive instead of current-prefix-arg

Currently notmuch-show looks at the prefix-arg directly via
current-prefix-arg. This changes it to use the interactive
specification.

One test (for elide-toggle functionality) set the prefix arg
directly. Update this test to set the new argument directly.
Place extra_cflags before CONFIGURE_CFLAGS

This ensures that the build will not attempt to use an existing notmuch.h when
an older version of notmuch is already installed elsewhere (e.g. in /usr/local)
and /usr/local/include is added to CONFIGURE_CFLAGS by one of the libraries
(talloc, in my case)
perf: Clean corpus and caches in distclean, not clean

Previously, we cleaned the downloaded performance corpus and the
cached indexes on 'make clean'.  This seems heavy-handed, since these
take a long time to download, unpack, and index.  They also aren't
make targets to begin with.  Move cleaning these to 'make distclean'.
This isn't exactly the right meaning of "distclean", but it's closer.
emacs: Fix misuse of `notmuch-tag'

The calling convention for `notmuch-tag' changed in commit 97aa3c06 to
take a list of tag changes instead of a &rest argument, but the call
from `notmuch-search-tag-all' still passed a &rest argument.  This
happened to work for interactive calls because tag-changes would be
nil, so the `apply' call would pass only the query string to
`notmuch-tag' and simply omit the &optional tag-changes argument.
emacs: Take prompt and current tags in `notmuch-read-tag-changes'

This modifies the interface of `notmuch-read-tag-changes' to take an
optional prompt string as well as a list of existing tags instead of a
query.  This list of tags is used to populate the tag removal
completions and lets the caller compute these in a more
efficient/consistent manner than performing a potentially large or
complex query.  This patch also updates the sole current caller of
`notmuch-read-tag-changes'.
emacs: Use interactive specifications for tag changes in show

This modifies all tagging operations in show to call
`notmuch-read-tag-changes' in their interactive specification to input
tag changes, rather than depending on lower-level functions to prompt
for tag changes regardless of their calling context.

Besides being more Elispy and providing a more consistent programmatic
API, this enables callers to provide two call site-specific pieces of
information: an appropriate prompt, and the set of visible tags.  The
prompt lets us differentiate * from +/-.  Providing visible tags
enables a more consistent user experience than retrieving the
(potentially different) tags from the database, and avoids a
round-trip to the CLI and database.
emacs: Use interactive specifications for tag changes in search

This is similar to the previous commit, but applies to search.

Search is somewhat more complicated because its tagging operations can
also apply to a region.  Hence, this lifts interactive prompting into
a helper function.  This also takes advantage of the new ability to
provide a prompt to distinguish tagging a single thread from tagging a
region of threads.
pick: Fix incorrect use of `notmuch-pick-tag'

`notmuch-pick-tag' takes a list of tag changes, but
`notmuch-pick-archive-message' passes it a &rest argument.  This
happens to work if `notmuch-archive-tags' contains a single tag (which
it usually does), but will break if it does not.
pick: Use list form of tag-changes in test

Currently we support both string and list forms of tag-changes for
historical reasons.  This is about to change, so fix pick's tests that
use the legacy string form of tag-changes.
pick: Use interactive specifications for tag changes
emacs: Remove interactive behavior of `notmuch-tag'

We no longer use this, since we've lifted all interactive behavior to
the appropriate interactive entry points.  Because of this,
`notmuch-tag' also no longer needs to return the tag changes list,
since the caller always passes it in.
emacs: Sanitize authors and subjects in search and show

Authors and subjects can contain embedded, encoded control characters
like "\n" and "\t" that mess up display.  Transform control characters
into spaces everywhere we display them in search and show.
emacs: Add a space after completed tag operations

Previously, when a user fully completed a tag operation, they had to
press space to begin entering another tag operation.  This is
different from, say, shell file name completion, which typically
inserts a space after an unambiguous completion under the assumption
that the user will want to enter more input.

This patch tweaks `notmuch-read-tag-changes' to act more like shell
file name completion: after an unambiguous tag completion, it now
inserts a space, ready and waiting for another tagging operation from
the user.  This is backwards-compatible with old habits, since there's
no harm in putting an extra space.
Merge branch 'master' of git://notmuchmail.org/git/notmuch