Handle an upstream branch for an unborn one

Skip tests on Clar

When the current branch is unborn, git will still mark the current
branch's upstream for-merge if there is an upstream configuration. The
only non-constrived case is cloning from an empty repository which then
gains history. origin's master should be marked for-merge.

In order to do this, we cannot use the high-level wrappers that expect a
reference, as we may not have one. Move over to the internal ones that
expect a reference name, which we do have.

Cloning from an empty repo must set master's upstream to origin's
master, even if neither of them exist.

Fetching from a non-empty origin must then mark the master branch
for-merge. This currently fails.

Fix submodule_is_config_only's return value

No need to find merge base.

Correct a stale reference to GIT_EBAREINDEX

Per @carlosmn, git_index_add is now named git_index_add_bypath.

Skip untracked contained repo contents even with gitlink files

One blame test replies on being run from within the libgit2
repository to leverage having a longer history to play with, but
some bundled versions of libgit2 don't have the whole libgit2
history.  This just skips that test if the repository can't be
opened.

When doing a diff for use in status, we should never show the
content of a git repository contained inside another one.  The
logic to do this was looking for a .git directory and so when a
gitlink plain .git file was used, it was failing to exclude the
directory content.

In-memory packing backend

Fix submodule sort order during iteration

Add CFLAGS -Wdeclaration-after-statement

This warns local variables declarations after statement, which helps not to break MSVC

Use git_object_short_id in examples

Rename in-memory remote to anonymous and swap url and fetch order

The order in this function is the opposite to what
create_with_fetchspec() has, so change this one, as url-then-refspec is
what git does.

As we need to break compilation and the swap doesn't do that, let's take
this opportunity to rename in-memory remotes to anonymous as that's
really what sets them apart.

Fix git_odb_short_id and git_odb_exists_prefix bugs

With the changes to how git_path_dirload_with_stat handles things
that look like submodules, submodules could end up sorted in the
wrong order with the workdir iterator.  This moves the submodule
check earlier in the iterator processing of a new directory so
that the submodule name updates will happen immediately and the
sort order will be correct.

Const up members of git_merge_file_result

Introduce git_merge_head_id

Don't reset need_pack

While looping over multiple heads, an up-to-date head will clobber the `remote->need_pack` setting, preventing the rest of the machinery from building and downloading a pack-file, breaking fetches.

Update clar to 4b75388

RFC: Sandbox HOME during test execution