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.

Minor code cleanups

Fix repo init when template dir is non-existent

Instead of using the project-specific `libgit2_SOURCE_DIR` and
`libgit2_BINARY_DIR` variables, use `CMAKE_SOURCE_DIR` and
`CMAKE_BINARY_DIR`.

The filebuf functions should use hashes directly, not indirectly
using the oid functions.

Remove some unnecessary includes from utility code.