remote-svn: add marks-file regeneration

remote-svn: add marks-file regeneration

fast-import mark files are stored outside the object database and are
therefore not fetched and can be lost somehow else.  marks provide a
svn revision --> git sha1 mapping, while the notes that are attached
to each commit when it is imported provide a git sha1 --> svn revision
mapping.

If the marks file is not available or not plausible, regenerate it by
walking through the notes tree.  , i.e.  The plausibility check tests
if the highest revision in the marks file matches the revision of the
top ref. It doesn't ensure that the mark file is completely correct.
This could only be done with an effort equal to unconditional
regeneration.

Signed-off-by: Florian Achleitner <florian.achleitner.2.6.31@gmail.com>
Acked-by: David Michael Barr <b@rr-dav.id.au>
Signed-off-by: Junio C Hamano <gitster@pobox.com>