Re: [PATCH v3 6/8] reset: make sparse-aware (except --mixed)

Junio C Hamano wrote:

For most callers of prime_cache_tree(), like the ones in "git
read-tree" and "git reset", it is immediately obvious that we just
read from the same tree, and we should have everything from the tree
and nothing else in the resulting index, so it is clear that the
prime_cache_tree() call is recreating the same cache-tree
information that we already should have computed ourselves, and
these calls can go (or if "prime" is still cheaper than "update",
these callers can pass an option to tell unpack_trees() to skip the
cache_tree_update() call, because they will call "prime" immediately
after).

After some basic performance testing of `git reset [--hard]`, it's not clear
whether `cache_tree_update` is definitively faster or slower than
`prime_cache_tree`; more conclusive results would indicate which of the two
could be skipped. I'd like to defer this to a future patch (tracking it with
an internal issue so I don't forget) where I can perform a more thorough
analysis across all of the commands currently using `prime_cache_tree` and
update its usage accordingly.