This is a sanity check on a bug I stumbled upon while writing unit tests
for error handling in the library.

The attached test script (meant to be run from within the test subdir of
a built source tree) demonstrates trying (and failing) to index a
relative path.

Am I missing something here, or does _notmuch_message_file_open_ctx (or
something that calls it) need to check for absolute paths and prepend
the mail root if not?

I'm just a bit surprised that no-one hit this before.