changectx: extract explicit computechangesetfilesremoved method from context
Right now, the logic around changeset centric removed files data are buried into
the "changectx" code. We extract this code in a dedicated method (in the scmutil
module) for clarity. This clarity will help to explicitly compute and caches
these data in the future.
#require docutils gettext
$ $TESTDIR/check-gendoc sv
checking for parse errors