configitems: add a new config option to control new filenode functionality
This series is adding functionality where we can force create a new filenode for
some files on a merge-commit. This is meant to represent an explicit choice made
by user and hence distingusing the file in the merged commit with the file in
parent by creating a new filenode instead of using the parent one.
This introduces a experimental config option under which this functionality
will be hidden.
A new testcase is added in tests/test-merge-criss-cross.t to test the new
functionality while making the option does not break other scenarios.
Differential Revision: https://phab.mercurial-scm.org/D9026
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