test-rebase-interruptions: stabilize for Windows

External hooks end up launching cmd.exe, which knows nothing about $VAR syntax.
For some reason, I thought that Mercurial would substitute in the value, in
order to paper over the platform difference.  But I can't find that in the
documentation, and there's at least one other use of this pattern [1].

[1] https://www.mercurial-scm.org/repo/hg/file/tip/tests/test-histedit-fold.t#l477

tests/test-rebase-interruptions.t

@@ -333,7 +333,12 b' Test rebase interrupted by hooks'
 
   $ cp -R a3 hook-pretxncommit
   $ cd hook-pretxncommit
-  $ hg rebase --source 2 --dest 5 --tool internal:other --config 'hooks.pretxncommit=hg log -r $HG_NODE | grep "summary:     C"'
+#if windows
+  $ NODE="%HG_NODE%"
+#else
+  $ NODE="\$HG_NODE"
+#endif
+  $ hg rebase --source 2 --dest 5 --tool internal:other --config "hooks.pretxncommit=hg log -r $NODE | grep \"summary:     C\""
   rebasing 2:965c486023db "C"
   summary:     C
   rebasing 6:a0b2430ebfb8 "F" (tip)