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>