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 history. 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