git-svn: rebuild convenience and bugfixes

We will now automatically fetch the refs/remotes/git-svn ref
from origin and store a Pull: line for it.

--remote=<origin> may be passed if your remote is named something
other than 'origin'

Also, remember to make GIT_SVN_DIR whenever we need to create
.rev_db

Signed-off-by: Eric Wong <normalperson@yhbt.net>