commitctx: rename "changed" to touched
The variable contains content that are both added and modified. "changed" could
be confused with "modified" only, so we pick a less ambiguous naming that will
help with more unification.
For example, it would make sense to shove the "removed" item in there since
this is how the variable is used.
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/D8706