sqlite: Blob type for oid field and libgit2 allocation

[gumenikav]

• Text field is not suitable for an oid data, because of an oid can contain null-symbols.
• The buffer should be allocated using the function git_odb_backend_malloc to ensure that it can be safely freed later
• Тhere is no need to copy data(SQLITE_TRANSIENT) since query to the database is done in the scope of the same function