t5510: run auto-gc in the foreground

t5510: run auto-gc in the foreground

The last test added to 't5510-fetch' in 0898c9628104 (fetch: release
pack files before garbage-collecting, 2016-01-13) may sporadically
trigger following error message from the test harness:

  rm: cannot remove 'trash directory.t5510-fetch/auto-gc/.git': Directory not empty

The test in question forces an auto-gc, which, if the system supports
it, runs in the background by default, and occasionally takes long
enough for the test to finish and for 'test_done' to start
housekeeping.  This can lead to the test's 'git gc --auto' in the
background and 'test_done's 'rm -rf $trash' in the foreground racing
each other to create and delete files and directories.  It might just
happen that 'git gc' re-creates a directory that 'rm -rf' already
visited and removed, which ultimately triggers the above error.

Disable detaching the auto-gc process to ensure that it finishes
before the test can continue, thus avoiding this racy situation.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>