packfile: skip hash checks in add_promisor_object()

When is_promisor_object() is called for the first time, it lazily initializes a set of all promisor objects by iterating through all objects in promisor packs. For each object, add_promisor_object() calls parse_object(), which decompresses and hashes the entire object. For repositories with large pack files, this can take an extremely long time. For example, on a production repository with a 176 GB promisor pack: $ time ~/git/git/git-rev-list --objects --all --exclude-promisor-objects --quiet ________________________________________________________ Executed in 76.10 mins fish external usr time 72.10 mins 1.83 millis 72.10 mins sys time 3.56 mins 0.17 millis 3.56 mins add_promisor_object() just wants to construct the set of all promisor objects, so it doesn't really need to verify the hash of every object. Set PARSE_OBJECT_SKIP_HASH_CHECK to skip the hash check. This has the side effect of skipping decompression of blob objects completely, saving a significant amount of time: $ time ~/git/git/git-rev-list --objects --all --exclude-promisor-objects --quiet ________________________________________________________ Executed in 124.70 secs fish external usr time 46.94 secs 0.00 millis 46.94 secs sys time 43.11 secs 1.03 millis 43.11 secs Signed-off-by: Aaron Plattner <aplattner@nvidia.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>