builtin/log: prefetch necessary blobs for `git cherry`

In partial clones, `git cherry` fetches necessary blobs on-demand one
at a time, which can be very slow.  We would like to prefetch all
necessary blobs upfront.  To do so, we need to be able to first figure
out which blobs are needed.

`git cherry` does its work in a two-phase approach: first computing
header-only IDs (based on file paths and modes), then falling back to
full content-based IDs only when header-only IDs collide -- or, more
accurately, whenever the oidhash() of the header-only object_ids
collide.

patch-ids.c handles this by creating an ids->patches hashmap that has
all the data we need, but the problem is that any attempt to query the
hashmap will invoke the patch_id_neq() function on any colliding objects,
which causes the on-demand fetching.

Insert a new prefetch_cherry_blobs() function before checking for
collisions.  Use a temporary replacement on the ids->patches.cmpfn
in order to enumerate the blobs that would be needed without yet
fetching them, and then fetch them all at once, then restore the old
ids->patches.cmpfn.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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