tests: introduce "checkeditform-n-cat.sh" script to invoke "cat" in it safely

Before this patch, a part of "test-transplant.t" fails unexpectedly on
Windows environment, because semicolon (";") in HGEDITOR isn't
recognized as the command separator by "cmd.exe".

This patch newly introduces "checkeditform-n-cat.sh" script to invoke
"cat" in it safely anywhere.

tests/test-transplant.t

@@ -99,7 +99,11 b" clone so subsequent rollback isn't affec"
   > env | grep HGEDITFORM
   > true
   > EOF
-  $ HGEDITOR="sh $TESTTMP/checkeditform.sh; cat" hg transplant --edit 7
+  $ cat > $TESTTMP/checkeditform-n-cat.sh <<EOF
+  > env | grep HGEDITFORM
+  > cat \$*
+  > EOF
+  $ HGEDITOR="sh $TESTTMP/checkeditform-n-cat.sh" hg transplant --edit 7
   applying ffd6818a3975
   HGEDITFORM=transplant.normal
   b3