Suppress reporting implicit git gc calls, when they don’t do anything

Hello,

With git 19.2.2 after `git pull` the output was
1. Autopacking the repository in background for optimal performance. 
See "git help gc" for manual housekeeping

2. error: Your local changes to the following files would be
overwritten by merge:
  …
Please commit your changes or stash them before you merge.
Aborting

This implies, that `git gc` runs in the background.

Then I called `git rebase --autostash` on which the output contained again:

“Autopacking the repository in background for optimal performance. See "git help gc" for manual housekeeping.”

The second message means, that `git gc` is again started in the background, before the first GC has finished. Clearly non-sense.

In practice, the second GC does not run, or exits very fast.

* Alter `git rebase --autostash` not to write, that git gc is called,
when that GC doesn’t do anything.

Regards
  Дилян