Also repurposed an unused function and deleted another one.

This change makes that anything that calls `git_diff__from_iterators`
(any of the `git_diff_xxx` functions) only need to parse the
`.gitmodules` file once. This can be avoided by calling
`git_repository_submodule_cache_all(...)`, but we can do that safely for
the user with no change in semantics.

Fixes: #5725

`git_buf_sanitize` is called with user-input, and wants to sanity-check
that input.  Allow it to return a value if the input was malformed in a
way that we cannot cope.

When compiling libgit2 with -DDEPRECATE_HARD, we add a preprocessor
definition `GIT_DEPRECATE_HARD` which causes the "git2/deprecated.h"
header to be empty. As a result, no function declarations are made
available to callers, but the implementations are still available to
link against. This has the problem that function declarations also
aren't visible to the implementations, meaning that the symbol's
visibility will not be set up correctly. As a result, the resulting
library may not expose those deprecated symbols at all on some platforms
and thus cause linking errors.

Fix the issue by conditionally compiling deprecated functions, only.
While it becomes impossible to link against such a library in case one
uses deprecated functions, distributors of libgit2 aren't expected to
pass -DDEPRECATE_HARD anyway. Instead, users of libgit2 should manually
define GIT_DEPRECATE_HARD to hide deprecated functions. Using "real"
hard deprecation still makes sense in the context of CI to test we don't
use deprecated symbols ourselves and in case a dependant uses libgit2 in
a vendored way and knows it won't ever use any of the deprecated symbols
anyway.

We've accumulated quite some functions which are never used outside of
their respective code unit, but which are lacking the `static` keyword.
Add it to reduce their linkage scope and allow the compiler to optimize
better.

The submodule code has grown out-of-date regarding its coding style.
Update `git_submodule_reload` and `git_submodule_sync` to more closely
resemble what the rest of our code base uses.

git_submodule_sync should resolve submodule before writing to .git/config
to have the same behavior as git_submodule_init, which does the right thing.