t6500: wait for detached auto gc at the end of the test script

t6500: wait for detached auto gc at the end of the test script

The last test in 't6500-gc', 'background auto gc does not run if
gc.log is present and recent but does if it is old', added in
a831c06a2 (gc: ignore old gc.log files, 2017-02-10), may sporadically
trigger an error message from the test harness:

  rm: cannot remove 'trash directory.t6500-gc/.git/objects': Directory not empty

The test in question ends with executing an auto gc in the backround,
which occasionally takes so long that it's still running when
'test_done' is about to remove the trash directory.  This 'rm -rf
$trash' in the foreground might race with the detached auto gc to
create and delete files and directories, and gc might (re-)create a
path that 'rm' already visited and removed, triggering the above error
message when 'rm' attempts to remove its parent directory.

Commit bb05510e5 (t5510: run auto-gc in the foreground, 2016-05-01)
fixed the same problem in a different test script by simply
disallowing background gc.  Unfortunately, what worked there is not
applicable here, because the purpose of this test is to check the
behavior of a detached auto gc.

Make sure that the test doesn't continue before the gc is finished in
the background with a clever bit of shell trickery:

  - Open fd 9 in the shell, to be inherited by the background gc
    process, because our daemonize() only closes the standard fds 0,
    1 and 2.
  - Duplicate this fd 9 to stdout.
  - Read 'git gc's stdout, and thus fd 9, through a command
    substitution.  We don't actually care about gc's output, but this
    construct has two useful properties:
  - This read blocks until stdout or fd 9 are open.  While stdout is
    closed after the main gc process creates the background process
    and exits, fd 9 remains open until the backround process exits.
  - The variable assignment from the command substitution gets its
    exit status from the command executed within the command
    substitution, i.e. a failing main gc process will cause the test
    to fail.

Note, that this fd trickery doesn't work on Windows, because due to
MSYS limitations the git process only inherits the standard fds 0, 1
and 2 from the shell.  Luckily, it doesn't matter in this case,
because on Windows daemonize() is basically a noop, thus 'git gc
--auto' always runs in the foreground.

And since we can now continue the test reliably after the detached gc
finished, check that there is only a single packfile left at the end,
i.e. that the detached gc actually did what it was supposed to do.
Also add a comment at the end of the test script to warn developers of
future tests about this issue of long running detached gc processes.

Helped-by: Jeff King <peff@peff.net>
Helped-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>