Deploy git_repository_set_head()

Diff: Show submodule diff

Fix a bug where ignorecase wasn't applied to ignores

The reference is only needed inside the function. We mistakenly
increased the reference counter causing the ODB not to get freed and
leaking descriptors.

Storing flushes in the refs vector doesn't let us recognize when the
remote is empty, as we'd always introduce at least one element into
it. These flushes aren't necessary, so we can simply ignore them.

We don't have anything useful that we could do with that oid anyway (We
need to query the submodule for the HEAD commit instead).

Without this, the following code creates the error "Failed to read
descriptor: Is a directory" when run against the submod2 test-case:

    const char* oidstr = "873585b94bdeabccea991ea5e3ec1a277895b698";
    git_tree* tree = resolve_commit_oid_to_tree(g_repo, oidstr);
    git_diff_list* diff = NULL;
    cl_assert(tree);
    cl_git_pass(git_diff_workdir_to_tree(g_repo, NULL, tree, &diff));

1. teach diff.c:maybe_modified to query git_submodule_status for the
   modification state of a submodule. According to the
   git_submodule_status docs, it will filter for to-ignore states
   already.

2. teach diff_output.c:get_workdir_content to check the submodule status
   again and create a line like:

      Subproject commit <SHA-1>\n
   or
      Subproject comimt <SHA-1>-dirty\n

   like git.git does.

diff_output.c:get_blob_content used to try to read the submodule commit
as a blob in the superproject's odb. Of course it cannot find it and
errors out with GIT_ENOTFOUND, implcitly terminating the whole diff
output.

This patch teaches it to create a text that describes the submodule
instead. The text looks like:

	Subproject commit <SHA1>\n

which is what git.git does, too.

ODB: re-load packfiles on failed lookup

Support for the core.ignorecase flag

Win64 cleanups

Include tag

Also honor remote.$name.tagopt = --tags.

The local function works for simple cases, but we shouldn't reinvent
the wheel just for us.

Together with include-tag, this make us behave more like git. After a
fetch, try to create any tags the remote told us about for which we
have objects locally.

This tells the remote to send us any tags that point to objects that
we are downloading.

Indicate whether the error comes from the ref already existing or
elsewhere. We always perform the check and this lets the user write
more concise code.

The latter shouldn't be exposed and isn't used, git_refspec__parse
supersedes it.

Fix a leak in the refspec tests while we're at it.