branch: use write_file_buf instead of write_file

branch: use write_file_buf instead of write_file

If we already have a strbuf, then using write_file_buf is a
little nicer to read (no wondering whether "%s" will eat
your NULs), and it's more efficient (no extra formatting
step).

We don't care about the newline magic of write_file(), as we
have our own multi-line content.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>