Use the new object_type enumeration names within the codebase.

submodule: add test initialising and cloning a repo

Instead of having it set in a different place from every other callback,
put it the main structure. This removes some state from the remote and
makes it behave more like clone, where the constructors are passed via
the options.

Having the setting be different from calling its actions was not a great
idea and made for the sake of the wrong convenience.

Instead of that, accept either fetch options, push options or the
callbacks when dealing with the remote. The fetch options are currently
only the callbacks, but more options will be moved from setters and
getters on the remote to the options.

This does mean passing the same struct along the different functions but
the typical use-case will only call git_remote_fetch() or
git_remote_push() and so won't notice much difference.

We have a few tests checking each step, but we do not yet have a test
which tests the documented workflow for creating a submodule, namely
`setup_add` followed by cloning into it, followed by `add_finalize`.

Add such a test to protect against regressions in this workflow.

When there is a tag, we must make sure that we get all referenced
objects from this tag as well. This failing test shows that e.g. when
there is a tagged tree, we insert the top tree but do not descend, thus
causing the clone to have broken links.

This changes the get_entry() method to return a refcounted version of
the config entry, which you have to free when you're done.

This allows us to avoid freeing the memory in which the entry is stored
on a refresh, which may happen at any time for a live config.

For this reason, get_string() has been forbidden on live configs and a
new function get_string_buf() has been added, which stores the string in
a git_buf which the user then owns.

The functions which parse the string value takea advantage of the
borrowing to parse safely and then release the entry.