test-amend: fix HGEDITOR shell script to run on Windows

Windows doesn't know how to launch *.sh directly.  This workaround is used in
several other tests.

tests/test-amend.t

@@ -118,10 +118,10 b' Amend with editor'
   > EOF
   $ chmod +x $TESTTMP/prefix.sh
 
-  $ HGEDITOR="$TESTTMP/prefix.sh" hg amend --edit -q
+  $ HGEDITOR="sh $TESTTMP/prefix.sh" hg amend --edit -q
   $ hg log -r . -T '{node|short} {desc}\n'
   298f085230c3 EDITED: NEWMESSAGE
-  $ HGEDITOR="$TESTTMP/prefix.sh" hg amend -e -m MSG -q
+  $ HGEDITOR="sh $TESTTMP/prefix.sh" hg amend -e -m MSG -q
   $ hg log -r . -T '{node|short} {desc}\n'
   974f07f28537 EDITED: MSG