t0021: use Windows path when appropriate

Since c6b0831c9c (docs: warn about possible '=' in clean/smudge filter
process values, 2016-12-03), t0021 writes out a file with quotes in its
name, and MSYS2's path conversion heuristics mistakes that to mean that
we are not talking about a path here.

Therefore, we need to use Windows paths, as the test-helper is a Win32
program that would otherwise have no idea where to look for the file.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t0021-conversion.sh

@@ -8,8 +8,8 @@ export GIT_TEST_DEFAULT_INITIAL_BRANCH_NAME
 . ./test-lib.sh
 . "$TEST_DIRECTORY"/lib-terminal.sh
 
-TEST_ROOT="$PWD"
-PATH=$TEST_ROOT$PATH_SEP$PATH
+TEST_ROOT="$(pwd)"
+PATH=$PWD$PATH_SEP$PATH
 
 write_script <<\EOF "$TEST_ROOT/rot13.sh"
 tr \