git svn info: make info relative to the current directory

git svn info: make info relative to the current directory

Previously 'git svn info <path>' would always treat the <path> as
relative to the working directory root, with a default of ".".  This
does not match the behaviour of 'svn info'.  Prepend $(git rev-parse
--show-prefix) to the path used inside cmd_info to make it relative to
the current working directory.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Acked-by: Eric Wong <normalperson@yhbt.net>