tests-racy-mutation: pass the editor through config instead of env variable

On Windows msys seems to do awful mangling of the environment variable content
that confuses everything to the death. Going through the config works fine, so
we do that instead.

Differential Revision: https://phab.mercurial-scm.org/D12479

tests/test-racy-mutations.t

@@ -38,14 +38,24 b' Start an hg commit that will take a whil'
   > EOF
 #endif
 
+  $ cat >> .hg/hgrc << EOF
+  > [ui]
+  > editor=sh $TESTTMP/waitlock_editor.sh
+  > EOF
+
   $ echo foo > foo
-  $ (WAITLOCK_ANNOUNCE="${EDITOR_STARTED}" \
+  $ (unset HGEDITOR;
+  >      WAITLOCK_ANNOUNCE="${EDITOR_STARTED}" \
   >      WAITLOCK_FILE="${MISCHIEF_MANAGED}" \
-  >           HGEDITOR="sh $TESTTMP/waitlock_editor.sh" \
   >           hg commit -qAm 'r1 (foo)' --edit foo > .foo_commit_out 2>&1 ; touch "${JOBS_FINISHED}") &
 
 Wait for the "editor" to actually start
-  $ WAITLOCK_FILE="${EDITOR_STARTED}" sh "$TESTTMP/waitlock_editor.sh"
+  $ sh "$RUNTESTDIR/testlib/wait-on-file" 5 "${EDITOR_STARTED}"
+
+  $ cat >> .hg/hgrc << EOF
+  > [ui]
+  > editor=
+  > EOF
 
 Break the locks, and make another commit.
   $ hg debuglocks -LW