Adding exclude tags to any query will show empty messages that are
tagged as spam. One has to explicitly search for tag:spam to actually
view them, but then the content of the messages should be shown.
So explicitly add exclude tags only to folders (count) and search list.
test/insert: add known broken tests for indexing failures

These tests are written with the assumption that we want all indexing
failures to be considered as failures by notmuch insert.
cli/insert: add fail path to add_file_to_database

Handle failures gracefully in add_file_to_database, renamed simply
add_file while at it. Add keep option to not remove the message from
database if tagging or tag syncing to maildir flags fails. Expand the
function documentation to cover the changes.
cli/insert: require succesful message indexing for success status

Add --keep option to keep any remaining stuff in index or file. We
could distinguish between failures to index and failures to apply tags
or maildir sync, but for simplicity just have one.
test/insert: check that indexing errors are accepted with --keep

This is overkill for the current code path, but should provide some
robustness for future changes in error handling.
test: check for gdb in insert tests

Commits a6cee01 and c48b12f otherwise break the build for people
without gdb.
Merge branch 'master' into vim-devel
VIM: syntax/notmuch-search
set tags highlighting to Comment. It usually gives a better contrast
between Normal and Comment.
Background: I am red/green colorblind. I now had two colorschemes -
solarized and luna - where I could not distinguish Subject and Tag.
This changes is due to personal preference, but it shouldn't hurt anyone.