In order to be able to write symlinks with git_blob_create_fromfile(),
we need to check whether the file to be written is a symbolic link or
not. Since the calling function of git_blob_create_fromfile() is likely to have
stated the file before calling, we make it pass the stat.

The reason for this is that writing symbolic link blobs is significantly
different from writing ordinary files - we do not want to open the link
destination but instead want to write the link itself, regardless of
whether it exists or not.

Previously, index_init_entry() used to error out if the file to be added
was a symlink that pointed to a nonexistent file. Fix this behaviour to
add the file regardless of whether it exists. This mimics git.git's
behaviour.