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