Add a svnrdump-simulator replaying a dump file for testing

Add a svnrdump-simulator replaying a dump file for testing

To ease testing without depending on a reachable svn server, this
compact python script mimics parts of svnrdumps behaviour.  It
requires the remote url to start with sim://.

Start and end revisions are evaluated.  If the requested revision
doesn't exist, as it is the case with incremental imports, if no new
commit was added, it returns 1 (like svnrdump).

To allow using the same dump file for simulating multiple incremental
imports, the highest revision can be limited by setting the environment
variable SVNRMAX to that value. This simulates the situation where
higher revs don't exist yet.

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>