test: use the "correct" merge tool in `test-rebase-detach.t`

The value of `HGMERGE` overrides the value passed through --config. We are
about to drop this `HGMERGE` environment variable so we update the value used
in config to match what is currently in use.

This is another example of confusion introduced by the `HGMERGE` environment
variable.

tests/test-rebase-detach.t

@@ -293,7 +293,7 b' Ensure --continue restores a correct sta'
   $ hg ci -A -m 'B2'
   adding B
   $ hg phase --force --secret .
-  $ hg rebase -s . -d B --config ui.merge=internal:fail
+  $ hg rebase -s . -d B --config ui.merge=internal:merge
   rebasing 3:17b4880d2402 "B2" (tip)
   merging B
   warning: conflicts while merging B! (edit, then use 'hg resolve --mark')