An obvious place to fill the tree cache is on write-tree, as we're
guaranteed to be able to fill in the whole tree cache.
The way this commit does this is not the most efficient, as we read the
root tree from the odb instead of filling in the cache as we go along,
but it fills the cache such that successive operations (and persisting
the index to disk) will be able to take advantage of the cache, and it
reuses the code we already have for filling the cache.
Filling in the cache as we create the trees would require some
reallocation of the children vector, which is currently not possible
with out pool implementation. A different data structure would likely
allow us to perform this operation at a later date.
Keeping the cache around after read-tree is only one part of the
optimisation opportunities. In order to share the cache between program
instances, we need to write the TREE extension to the index.
Do so, taking the opportunity to rename 'entries' to 'entry_count' to
match the name given in the format description. The included test is
rather trivial, but works as a sanity check.
When reading from a tree, we know what every tree is going to look like,
so we can fill in the tree cache completely, making use of the index for
modification of trees a lot quicker.
This simplifies freeing the entries quite a bit; though there aren't
that many failure paths right now, introducing filling the cache from a
tree will introduce more. This makes sure not to leak memory on errors.
If there have been no pushes, we can immediately return ITEROVER. If
there have been no hides, we must not run the uninteresting pre-mark
phase, as we do not want to hide anything and this would simply cause us
to spend time loading objects.
This introduces a phase at the start of preparing a walk which pre-marks
uninteresting commits, but only up to the common ancestors.
We do this in a similar way to git, by walking down the history and
marking (which is what we used to do), but we keep a time-sorted
priority queue of commits and stop marking as soon as there are only
uninteresting commits in this queue.
This is a similar rule to the one used to find the merge-base. As we
keep inserting commits regardless of the uninteresting bit, if there are
only uninteresting commits in the queue, it means we've run out of
interesting commits in our walk, so we can stop.
The old mark_unintesting() logic is still in place, but that stops
walking if it finds an already-uninteresting commit, so it will stop on
the ones we've pre-marked; but keeping it allows us to also hide those
that are hidden via the callback.
