commitctx: extract the function in a dedicated module the function have few callers (< 15) is quite long a mostly independent from the repository itself. It seems like a good candidate to reduce the bloatness of the localrepository class. Extracting it will help us cleaning the code up and splitting it into more reasonable-size function. We don't use a copy trick because the amount of code extract is quite small (<5%) and the de-indent means every single line change anyway. So this is not deemed valuable to do so. This is part of a larger refactoring/cleanup of the commitctx code to clarify and augment the logic gathering metadata useful for copy tracing. The current code is a tad too long and entangled to make such update easy. Differential Revision: https://phab.mercurial-scm.org/D8709