test-histedit-fold: fix for Windows

Fixes 336121088ef1 failing with MSYS.

MSYS sh.exe mangled that path by applying its path magic, which caused the
test to fail on Windows.

tests/test-histedit-fold.t

@@ -208,7 +208,7 b' dropped revision.'
   |
   o  0:0189ba417d34 1+2+3
   
-  $ EDITED=`pwd`/../editcommands
+  $ EDITED="$TESTTMP/editcommands"
   $ cat > $EDITED <<EOF
   > pick 617f94f13c0f 1 +4
   > drop 888f9082bf99 2 +5