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.

The old separation was due to the old merge-base finding, so it's no
longer necessary.

These store merge-base information which is only valid for a single run.

hashsig: Export as a `sys` header

Since this isn't under sys/ we need to have it included in the main
git2.h file.

This fixes #2551.

Stash ignored directories

Pool/Index data is not aligned

Correctly handle getaddrinfo return result

global: free the error message when exiting a thread

* Ensure alignment by copying the content into a structure on the stack

Remove unused buf variable from path/core test.

The getaddrinfo function indicates failure with a non-zero return code,
but this code is not necessarily negative. On platforms like Android
where the code is positive, a failed call causes libgit2 to segfault.

Factor 40 and 41 constants from source.

It looks like what Travis wants is a list of hashes, rather than a hash
with a list.

Fix `git_path_walk_up` to work with non-rooted paths

Replace void casts with GIT_UNUSED.

Provide a callback for certificate validation