When "git svn reset" is called with an invalid revision, we
bail out and show the user a proper error message instead
of giving them a cryptic one related to git-svn internals.
ref: http://bugs.debian.org/578908 Signed-off-by: Jonathan Nieder <jrnieder@gmail.com> Reported-by: Jens Seidel <jensseidel@users.sf.net> Acked-by: Eric Wong <normalperson@yhbt.net>