search: use boolean prefixes for git blob queries

search: use boolean prefixes for git blob queries

I've hit some case where probabilistic searches don't work when
using dfpre:/dfpost:/dfblob: search prefixes because stemming in
the query parser interferes.

In any case, our indexing code indexes longer/unabbreviated blob
names down to its 7 character abbreviation, so there should be
no need to do wildcard searches on git blob names.