vcs-svn: use higher mark numbers for blobs

vcs-svn: use higher mark numbers for blobs

Prepare to use mark :5 for the commit corresponding to r5 (and so on).

1 billion seems sufficiently high for blob marks to avoid conflicting
with rev marks, while still leaving room for 3 billion blobs.  Such
high mark numbers cause trouble with ancient fast-import versions, but
this topic cannot support git fast-import versions before 1.7.4 (which
introduces the cat-blob command) anyway.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>