packfile: refactor `install_packed_git()` to work on packfile store
The `install_packed_git()` functions adds a packfile to a specific
object store. Refactor it to accept a packfile store instead of a
repository to clarify its scope.
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
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