gc: do not return error for prior errors in daemonized mode

gc: do not return error for prior errors in daemonized mode

Some build machines started consistently failing to fetch updated
source using "repo sync", with error

  error: The last gc run reported the following. Please correct the root cause
  and remove /build/.repo/projects/tools/git.git/gc.log.
  Automatic cleanup will not be performed until the file is removed.

  warning: There are too many unreachable loose objects; run 'git prune' to remove them.

The cause takes some time to describe.

In v2.0.0-rc0~145^2 (gc: config option for running --auto in
background, 2014-02-08), "git gc --auto" learned to run in the
background instead of blocking the invoking command.  In this mode, it
closed stderr to avoid interleaving output with any subsequent
commands, causing warnings like the above to be swallowed; v2.6.3~24^2
(gc: save log from daemonized gc --auto and print it next time,
2015-09-19) addressed that by storing any diagnostic output in
.git/gc.log and allowing the next "git gc --auto" run to print it.

To avoid wasteful repeated fruitless gcs, when gc.log is present, the
subsequent "gc --auto" would die after printing its contents.  Most
git commands, such as "git fetch", ignore the exit status from "git gc
--auto" so all is well at this point: the user gets to see the error
message, and the fetch succeeds, without a wasteful additional attempt
at an automatic gc.

External tools like repo[1], though, do care about the exit status
from "git gc --auto".  In non-daemonized mode, the exit status is
straightforward: if there is an error, it is nonzero, but after a
warning like the above, the status is zero.  The daemonized mode, as a
side effect of the other properties provided, offers a very strange
exit code convention:

 - if no housekeeping was required, the exit status is 0

 - the first real run, after forking into the background, returns exit
   status 0 unconditionally.  The parent process has no way to know
   whether gc will succeed.

 - if there is any diagnostic output in gc.log, subsequent runs return
   a nonzero exit status to indicate that gc was not triggered.

There's nothing for the calling program to act on on the basis of that
error.  Use status 0 consistently instead, to indicate that we decided
not to run a gc (just like if no housekeeping was required).  This
way, repo and similar tools can get the benefit of the same behavior
as tools like "git fetch" that ignore the exit status from gc --auto.

Once the period of time described by gc.pruneExpire elapses, the
unreachable loose objects will be removed by "git gc --auto"
automatically.

[1] https://gerrit-review.googlesource.com/c/git-repo/+/10598/

Reported-by: Andrii Dehtiarov <adehtiarov@google.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>