vcs-svn: add fast_export_note to create notes

vcs-svn: add fast_export_note to create notes

fast_export lacked a method to writes notes to fast-import stream.
Add two new functions fast_export_note which is similar to
fast_export_modify. And also add fast_export_buf_to_data to be able to
write inline blobs that don't come from a line_buffer or from delta
application.

To be used like this:

  fast_export_begin_commit("refs/notes/somenotes", ...)
  fast_export_note("refs/heads/master", "inline")
  fast_export_buf_to_data(&data)

or maybe

  fast_export_note("refs/heads/master", sha1)

Signed-off-by: Dmitry Ivankov <redacted>
Signed-off-by: Florian Achleitner <redacted>
Acked-by: David Michael Barr <redacted>
Signed-off-by: Junio C Hamano <redacted>