t3600: fix broken "choking git rm" test

t3600: fix broken "choking git rm" test

The test 'choking "git rm" should not let it die with cruft' is
supposed to check 'git rm's behavior when interrupted by provoking a
SIGPIPE while 'git rm' is busily deleting files from a specially
crafted index.

This test is silently broken for the following reasons:

- The test crafts a special index by feeding a large number of index
  entries with null shas to 'git update-index --index-info'.  It was
  OK back then when this test was introduced in commit 0693f9ddad
  (Make sure lockfiles are unlocked when dying on SIGPIPE,
  2008-12-18), but since commit 4337b5856f (do not write null sha1s to
  on-disk index, 2012-07-28) null shas are not allowed in the on-disk
  index causing 'git update-index' to error out.

- The barfing 'git update-index --index-info' should fail the test,
  but it remains unnoticed because of the severely broken && chain:
  the test's result depends solely on whether there is a stale lock
  file left behind, but after 'git update-index' errors out 'git rm'
  won't be executed at all.

To fix this test feed only non-null shas to 'git update-index' and
restore the && chain (partly by adding a missing && and by using the
test_when_finished helper instead of manual cleanup).

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>