While commit 9f673f94 (gc: config option for running --auto in
background - 2014-02-08) helps reduce some complaints about 'gc
--auto' hogging the terminal, it creates another set of problems.

The latest in this set is, as the result of daemonizing, stderr is
closed and all warnings are lost. This warning at the end of cmd_gc()
is particularly important because it tells the user how to avoid "gc
--auto" running repeatedly. Because stderr is closed, the user does
not know, naturally they complain about 'gc --auto' wasting CPU.

Daemonized gc now saves stderr to $GIT_DIR/gc.log. Following gc --auto
will not run and gc.log printed out until the user removes gc.log.
Signed-off-by: NNguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>