Trim while normalizing namespace for interwiki links

 * Fixes a lurking bug there (pointed out by marcoil). The normalized
   target may have changed length, so basing a substring of it off the
   initial prefix is error prone.

Bug: 68090
Change-Id: I3328de950d3473430ef6011b993230e60fe10404
Merge "Trim while normalizing namespace for interwiki links"