contrib/git-svn: fix svn compat and fetch args

contrib/git-svn: fix svn compat and fetch args

'svn info' doesn't work with URLs in svn <= 1.1.  Now we
only run svn info in local directories.

As a side effect, this should also work better for 'init' off
directories that are no longer in the latest revision of the
repository.

svn checkout -r<revision> arguments are fixed.
Newer versions of svn (1.2.x) seem to need URL@REV as well as
-rREV to checkout a particular revision...

Add an example in the manpage of how to track directory that has
been moved since its initial revision.

A huge thanks to Yann Dirson for the bug reporting and testing
my original patch.  Thanks also to Junio C Hamano for suggesting
a safer way to use git-rev-parse.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <junkio@cox.net>