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.

Provide a callback for certificate validation

This option make it easy to ignore anything about the server we're
connecting to, which is bad security practice. This was necessary as we
didn't use to expose detailed information about the certificate, but now
that we do, we should get rid of this.

If the user wants to ignore everything, they can still provide a
callback which ignores all the information passed.

If the certificate validation fails (or always in the case of ssh),
let the user decide whether to allow the connection.

The data structure passed to the user is the native certificate
information from the underlying implementation, namely OpenSSL or
WinHTTP.

When using a bare repo with an index, libgit2 attempts to read
files from the index.  It caches those files based on the path
to the file, specifically the path to the directory that contains
the file.

If there is no working directory, we use `git_path_dirname_r` to
get the path to the containing directory.  However, for the
`.gitattributes` file in the root of the repository, this ends up
normalizing the containing path to `"."` instead of the empty
string and the lookup the `.gitattributes` data fails.

This adds a test of attribute lookups on bare repos and also
fixes the problem by simply rewriting `"."` to be `""`.

 * Move the transport registration mechanisms into a new header under
   'sys/' because this is advanced stuff.
 * Remove the 'priority' argument from the registration as it adds
   unnecessary complexity.  (Since transports cannot decline to operate,
   only the highest priority transport is ever executed.)  Users who
   require per-priority transports can implement that in their custom
   transport themselves.
 * Simplify registration further by taking a scheme (eg "http") instead
   of a prefix (eg "http://").

Configuration snapshotting

Diff and status do not want core.safecrlf to actually raise an
error regardless of the setting, so this extends the filter API
with an additional options flags parameter and adds a flag so that
filters can be applied with GIT_FILTER_OPT_ALLOW_UNSAFE, indicating
that unsafe filter application should be downgraded from a failure
to a warning.

There were a couple of "init_opts()" functions a few more cases
of structure initialization that I somehow missed.