tests: avoid cryptic nodeids in tests/test-rename-merge1.t
These two nodeids had not been part of any output before, so one can't
know which revision they refer to without adding something like `hg
log` before them. It turned out that '.^' was equivalent for both of
them, so that's what I replaced them with.
Differential Revision: https://phab.mercurial-scm.org/D6280