omindex should allow indexing files it doesn't understand

[Olly Betts]

Users may want to index just the filenames and other filesystem metadata for files which omindex doesn't have a filter for.

Should be a compatible change, so marking as 1.2.x.

[Olly Betts]

This post has a suggested change to allow this:

​http://article.gmane.org/gmane.comp.search.xapian.general/8598

But with the recent omindex changes, it might be cleaner to add a way to say "don't try to extract text from this MIME content-type". With trunk, I think this should actually already work (I've not actually tested it though):

-Mjpg:image/jpeg -Mjpeg:image/jpeg -Fimage/jpeg:/bin/true

But it's not the most obvious interface, and we want to avoid running /bin/true in a subshell for each such file.

[Olly Betts]

Implemented a special case for true and documented this in [99e02ae01df059805e5e0265865ce7c4a568c59f].