commitctx: move a tiny else clause above the very long one This is simple to have all the simple case unfold before the 100+ line one. Otherwise it is hard to relate the `else` to the initial conditionnal. 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. We start with easy and small cleanup. Differential Revision: https://phab.mercurial-scm.org/D8708