This issue has been split out from issue2550583 (xapian search yields 
too few results), see b) in msg4059.

A search for "silent" does not match "silently" like it should.
The proposed solution is to chance the code to feed xapian only lower 
case words. The lower case words is what the stemming algorithm is
able to work with. The documentation of this within Xapian is not 
as clear as it could be.