Trivial fixups

Introduce reference transactions

Implement opportunistic ref updates

This leaves space for future expansion to locking other resources
without having to change the API for references.

Add `git_merge_bases_many`

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.

The stash is implemented as the refs/stash reference and its reflog. In
order to modify the reflog, we need avoid races by making sure we're the
only ones allowed to modify the reflog.

We achieve this via the transactions API. Locking the reference gives us
exclusive write access, letting us modify and write it without races.

A transaction allows you to lock multiple references and set up changes
for them before applying the changes all at once (or as close as the
backend supports).

This can be used for replication purposes, or for making sure some
operations run when the reference is locked and thus cannot be changed.

This will be used by the transaction code.

When a list of refspecs is passed to fetch (what git would consider
refspec passed on the command-line), we not only need to perform the
updates described in that refspec, but also update the remote-tracking
branch of the fetched remote heads according to the remote's configured
refspecs.

These "fetches" are not however to be written to FETCH_HEAD as they
would be duplicate data, and it's not what the user asked for.

The configured/base fetch refspecs need to be taken into account in
order to implement opportunistic remote-tracking branch updates. DWIM
them and store them in the struct, but don't do anything with them yet.

We can only DWIM when we've connected to the remote and have the list of
the remote's references. Adding or setting the refspecs should not
trigger an attempt to DWIM the refspecs as we typically cannot do it,
and even if we did, we would not use them for the current fetch.

This lets us test this bit as well as getting closer to what they were
trying to do.

With opportunistic ref updates, git has introduced the concept of having
base refspecs *and* refspecs that are active for a particular fetch.

Let's start by letting the user override the refspecs for download.

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.