[PATCH 2 of 2 evolve-ext] test: explicitly run HGEDITOR shell command with `sh`

[Matt Harbison]

# HG changeset patch
# User Matt Harbison <matt_harbison at yahoo.com>
# Date 1516571702 18000
#      Sun Jan 21 16:55:02 2018 -0500
# Node ID 0d16f38a2290784a99cc21eaf676b8a983144c13
# Parent  0fde72742b31666e742823cbb5fa2540a068fd89
test: explicitly run HGEDITOR shell command with `sh`

Otherwise Windows pops up the "What program do you want to use to open this?"
dialog.

diff --git a/tests/test-prev-next.t b/tests/test-prev-next.t
--- a/tests/test-prev-next.t
+++ b/tests/test-prev-next.t
@@ -243,7 +243,7 @@
 
   $ hg init repo
   $ cd repo
-  $ HGEDITOR=${TESTDIR}/fake-editor.sh
+  $ HGEDITOR="sh ${TESTDIR}/fake-editor.sh"
   $ echo hi > foo
   $ hg ci -Am 'one'
   adding foo