t0061: simplify .bat test

t0061: simplify .bat test

The test added by 71f4960b91 (t0061: fix test for argv[0] with spaces
(MINGW only), 2019-10-01) checks that we can use a .bat file with spaces
as GIT_SSH.

This is a good test in the sense that it's how the original bug was
detected. And as the commit message there describes, there are some
elements of the bug that are likely to come up with GIT_SSH and not
elsewhere: namely that in addition to the .bat file having spaces, we
must pass an argument with spaces (which happens naturally with ssh,
since we pass the upload-pack shell command for the other side to run).

But using GIT_SSH does complicate matters:

  1. We actually run the ssh command _twice_, once to probe the ssh
     variant with "-G" in fill_ssh_args(), and then a second time to
     actually make the connection. So we have to account for that when
     checking the output.

  2. Our fake ssh .bat file does not actually run ssh. So we expect the
     command to fail, but not before the .bat file has touched the "out"
     marker file that tells us it has run.

     This works now, but is fragile. In particular, the .bat file by
     default will echo commands it runs to stdout. From the perspective
     of the parent Git process, this is protocol-breaking garbage, and
     upon seeing it will die().

     That is OK for now because we don't bother to do any cleanup of the
     child process. But there is a patch under discussion, dd3693eb08
     (transport-helper, connect: use clean_on_exit to reap children on
     abnormal exit, 2026-03-12), which causes us to kill() the .bat
     process. This happens before it actually touches the "out" file,
     causing the test to fail.

We can simplify this by just using the "test-tool run-command" helper.
That lets us run whatever command we like with the arguments we want.
The argument here has a space, which is enough to trigger the original
bug that 71f4960b91 was testing. I verified that by reverting eb7c786314
(mingw: support spawning programs containing spaces in their names,
2019-07-16), the original fix, and confirming that the test fails (but
succeeds without the revert).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>