If a symbolic reference points to something invalid, then do not try to
update it.

`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.

Provide a function that can check remote name validity but can also
signal when an error occurs.  Use the name "name_is_valid", which is
more suggestive of checking a given name, rather than "is_valid_name",
which suggests that the function checks the validity of the current
remote's name.

Disambiguate invalid specifications in `git_refspec__parse` so that
callers can determine the difference between invalid specifications and
actual errors.  No call sites wil propagagte this new error message to
an end-user, so there is no user-facing API change.

When the remote does not tell us its default, we have to guess what the
default branch should be.  Use our local initial branch configuration to
inform the remote branch default when we clone.

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.