debugcommands: fix a missing b prefix
This wasn't causing any immediate problems because of the source
transformer, but I noticed the inconsistency and it bugged me.
# skip-blame just a b prefix
Differential Revision: https://phab.mercurial-scm.org/D4255
pager.pager=p0 # $TESTTMP/sysrc:4
ui.editor=e1 # $TESTTMP/userrc:2
pager.pager=p2 # $PAGER
ui.editor=e1 # $TESTTMP/userrc:2