builtin/gc: move rerere garbage collection into separate function
In a subsequent commit we are going to introduce a new "rerere-gc" task
for git-maintenance(1). To prepare for this, refactor the code that
spawns `git rerere gc` into a separate function.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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