commitctx: extract the function that commit a new manifest The logic is large enough and isolated enough to be extracted, this reduce the size of the main function, making it simpler to follow.

commitctx: stop using weakref proxy for transaction This weakref proxy was introduced in 2007 by 30d4d8985dd8. If I understand it correctly, the logic at that time was relying on the transaction destructor, triggered at garbage collection time to rollback failed transaction. passing the object to sub function directly mean it would live in the function scope and be trapped in the traceback on exception, leading to the transaction rollback too late in some case. Modern transaction usage use explicit opening and closing of transaction and no longer rely on some internal reference counting details. So this weakref proxy is no longer necessary. Absolutely no test are affected when we drop it.

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