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

mercurial/configitems.py

@@ -590,6 +590,11 b' coreconfigitem('
 coreconfigitem(
     b'experimental', b'maxdeltachainspan', default=-1,
 )
+# tracks files which were undeleted (merge might delete them but we explicitly
+# kept/undeleted them) and creates new filenodes for them
+coreconfigitem(
+    b'experimental', b'merge-track-salvaged', default=False,
+)
 coreconfigitem(
     b'experimental', b'mergetempdirprefix', default=None,
 )

tests/test-merge-criss-cross.t

@@ -1,3 +1,15 b''
+#testcases old newfilenode
+
+#if newfilenode
+Enable the config option
+------------------------
+
+  $ cat >> $HGRCPATH <<EOF
+  > [experimental]
+  > merge-track-salvaged = True
+  > EOF
+#endif
+
 Criss cross merging
 
   $ hg init criss-cross