rebase: fix regression in file change detection introduced by 0ecb3b11fcad
Before 0ecb3b11fcad, wctx._compact(), was called by wctx.nofilechanges() as
a side effect. Later, it turned out that this side effect is needed to correctly
detect which files changed. See https://phab.mercurial-scm.org/D8733#131949 for
The problem could also be triggered by running tests/test-rebase-parameters.t
with --extra-config-opt rebase.experimental.inmemory=1.
Differential Revision: https://phab.mercurial-scm.org/D8843