Extend the `git_worktree_add_options` to include `git_checkout_options`.

github issue #5949

Introduce `git_blob_data_is_binary` to examine a blob's data, instead of
the blob itself.  A replacement for `git_buf_is_binary`.

Users may want to validate raw object content; provide them a function
to do so.

Return `GIT_EINVALID` on parse errors so that direct callers of parse
functions can determine when there was a failure to parse the object.

The object parser functions will swallow this error code to prevent it
from propagating down the chain to end-users.  (`git_merge` should not
return `GIT_EINVALID` when a commit it tries to look up is not valid,
this would be too vague to be useful.)

The only public function that this affects is
`git_signature_from_buffer`, which is now documented as returning
`GIT_EINVALID` when appropriate.

To accommodate less capable compilers.

Co-authored-by: Peter Pettersson <85582231+boretrk@users.noreply.github.com>

Looking up a non-existent object currently always invokes
`git_odb_refresh`. If looking up a large batch of objects, many of which
may legitimately not exist, this will repeatedly refresh the ODB to no
avail.

Add a `git_odb_exists_ext` that accepts flags controlling the ODB
lookup, and add a flag to suppress the refresh. This allows the user to
control if and when they refresh (for instance, refreshing once before
starting the batch).

Commit b1a6c316a6070fac4ab1ec5792979838f7145c39 moved auto-refresh into
the pack backend, and added a comment accordingly. Commit
43820f204ea32503b4083e3b6b83f30a0a0031c9 moved auto-refresh back *out*
of backends into the ODB layer, but didn't update the comment.