pack-bitmap: iterate object sources when opening bitmaps

When opening a bitmap for a repository we perform two steps: - We first look for a multi-pack index bitmap in any of the object sources connected to the repository. - We then look for a packfile bitmap in any of the packfiles of any of the object sources. Both of these steps thus iterate through object sources themselves, one via `odb_prepare_alternates()` and one via `repo_for_each_pack()`. This layout makes it hard to introduce a way to open the bitmap of one specific object source, which is functionality that we'll require in a subsequent commit. Reverse the loop so that we instead loop through all sources in the outer loop, and then for each source we try to load its bitmap via either the multi-pack index or via a packfile. Note that this changes the precedence of bitmaps in one specific edge case: when an earlier object source only has a packfile bitmap, but a later source has a multi-pack index bitmap, we now pick the packfile bitmap of the earlier source. Previously, a multi-pack index bitmap from any source would have taken precedence over all packfile bitmaps. Given that object sources are ordered such that the local source comes first, this arguably is an improvement, as we now prefer local bitmaps over bitmaps in alternates. Furthermore, we already warn about repositories that have multiple bitmaps, so this setup is broken and thus arguably not worth worrying about too much. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>