git-svn: add a show-externals command.

git-svn: add a show-externals command.

show-externals can be used by scripts to provide svn:externals-like
functionality.  For example, a script can list all of the externals and then
use check out the listed URLs at the appropriate paths, similar to what the svn
client does.  Said script (or perhaps git-svn itself, in the future) could
simply invoke svn export on the paths, or it could go one further, using
git-svn clone and even git-submodule together to better integrate externals
checkouts.

The implementation is shamelessly copied from show-ignores.  A more general
command to list user-specified properties is probably a better idea.

Signed-off-by: Vineet Kumar <vineet@doorstop.net>
Acked-by: Eric Wong <normalperson@yhbt.net>