Our file utils functions all have a "futils" prefix, e.g.
`git_futils_touch`. One would thus naturally guess that their
definitions and implementation would live in files "futils.h" and
"futils.c", respectively, but in fact they live in "fileops.h".

Rename the files to match expectations.

Use the new object_type enumeration names within the codebase.

Test that `git_submodule_update` can handle a submodule that is
freshly cloned and has a path differing from its name.

Fallback describes the mechanism, while unspecified explains what the
user is thinking.

This lets us specify in the status call which ignore rules we want to
use (optionally falling back to whatever the submodule has in its
configuration).

This removes one of the reasons for having `_set_ignore()` set the value
in-memory. We re-use the `IGNORE_RESET` value for this as it is no
longer relevant but has a similar purpose to `IGNORE_FALLBACK`.

Similarly, we remove `IGNORE_DEFAULT` which does not have use outside of
initializers and move that to fall back to the configuration as well.

As submodules are becomes more like values, we should not let a status
check to update its properties. Instead of taking a submodule, have
status take a repo and submodule name.

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 want to use the "checkout: moving from ..." message in order to let
git know when a change of branch has happened. Make the convenience
functions for this goal write this message.

The signature for the reflog is not something which changes
dynamically. Almost all uses will be NULL, since we want for the
repository's default identity to be used, making it noise.

In order to allow for changing the identity, we instead provide
git_repository_set_ident() and git_repository_ident() which allow a user
to override the choice of signature.

This introduces the functionality of submodule update in
'git_submodule_do_update'. The existing 'git_submodule_update' function is
renamed to 'git_submodule_update_strategy'. The 'git_submodule_update'
function now refers to functionality similar to `git submodule update`,
while `git_submodule_update_strategy` is used to get the configured value
of submodule.<name>.update.