tests: adjust merge tool config in test-largefiles-update.t

Previously, the `ui.merge=internal:fail` configuration was shadowed by the the
`HGMERGE=internal:merge` environment variable. We would like to remove this
usage of `HGMERGE` variable because it is error-prone (eg: this very test). We
start by cleaning up test independently to avoid large churn when the change
happens.

tests/test-largefiles-update.t

@@ -3,9 +3,10
 This file focuses mainly on updating largefiles in the working
 directory (and ".hg/largefiles/dirstate")
 
+  $ unset HGMERGE
   $ cat >> $HGRCPATH <<EOF
   > [ui]
-  > merge = internal:fail
+  > merge = internal:merge
   > [extensions]
   > largefiles =
   > [extdiff]