revset: rename generationsrel() to generationssubrel()
This highlights that this is an implementation of 'x#y[z]' relation subscript
operator as opposed to 'x#y' relation operator.
Differential Revision: https://phab.mercurial-scm.org/D8393
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2