Re: [PATCH] Update "gc" behavior in commit, merge, am, rebase and index-pack

Jeff King [off-list ref] writes:

The pre-auto-gc hook runs only when we need to gc (i.e., when we would
be doing something expensive, anyway). So it is still cheap to check
whether we need to gc (although if you have an option like "--check"
which does not _fix_ the situation after checking, then you may end up
running the hook repeatedly).

Whether we need to gc might be cheap to check, but I think the worry about
the new hook added after commit lost auto-gc was the hook to see if the
user declines our offer to run "gc --auto" may be expensive with an extra
fork and exec.  But I am personally fine with "commit" running "gc --auto"
every time; pre-auto-gc hook is opt-in after all.

As you mentioned in your own follow-up message, however, it will make
things worse without any real benefit if it is merely "check and warn but
never gc".  Also when "gc --auto" does not make the repository better (I
vaguely recall some corner cases mentioned here in the past), repeated
invocation of it might make such a change annoying.