t9020: use configured Python to run the test helper

The test helper svnrdump_sim.py is used as "svnrdump" during the
execution of this test, but the arrangement was not optimal:

 - it relied on symbolic links;
 - unportable "export VAR=VAL" was used;
 - GIT_BUILD_DIR variable was not quoted correctly;
 - it assumed that the Python interpreter is in /usr/bin/ and
   called "python" (i.e. not "python2.7" etc.)

Rework this by writing a small shell script that spawns the right
Python interpreter, using the right quoting.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t9020-remote-svn.sh

@@ -12,9 +12,13 @@ then
 	test_done
 fi
 
-# We override svnrdump by placing a symlink to the svnrdump-emulator in .
-export PATH="$HOME:$PATH"
-ln -sf $GIT_BUILD_DIR/contrib/svn-fe/svnrdump_sim.py "$HOME/svnrdump"
+# Override svnrdump with our simulator
+PATH="$HOME:$PATH"
+export PATH PYTHON_PATH GIT_BUILD_DIR
+
+write_script "$HOME/svnrdump" <<\EOF
+exec "$PYTHON_PATH" "$GIT_BUILD_DIR/contrib/svn-fe/svnrdump_sim.py" "$@"
+EOF
 
 init_git () {
 	rm -fr .git &&