test-merge-tools: fix flaky test by avoiding debugsetparents

debugsetparents is a debug command and does not provide the same
guarantees as non-debug commands do. In particular, when the user sets
a different parent, any clean files will remain clean in the dirstate
even though the new parent might have a different version of the file
(so it should appear modified compared to the new parent). Let's
instead achieve the same effect by updating to the new parent and
reverting the contents back to what they were.

This fix can be tested by passing '--config
debug.dirstate.delaywrite=2' to the 'hg update' command in the
beforemerge().

tests/test-merge-tools.t

@@ -603,7 +603,8 b' update is a merge ...'
   true.priority=1
   true.executable=cat
   # hg update -C 1
-  $ hg debugsetparent 0
+  $ hg update -q 0
+  $ hg revert -q -r 1 .
   $ hg update -r 2
   merging f
   revision 1
@@ -628,7 +629,8 b' update should also have --tool'
   true.priority=1
   true.executable=cat
   # hg update -C 1
-  $ hg debugsetparent 0
+  $ hg update -q 0
+  $ hg revert -q -r 1 .
   $ hg update -r 2 --tool false
   merging f
   merging f failed!