git-svn: correctly access repos when only given partial read permissions

git-svn: correctly access repos when only given partial read permissions

Sometimes users are given only read access to a subtree inside a
repository, and git-svn could not read log information (and thus
fetch commits) when connecting a session to the root of the
repository.  We now start an SVN::Ra session with the full URL
of what we're tracking, and not the repository root as before.

This change was made much easier with a cleanup of
repo_path_split() usage as well as improving the accounting of
authentication batons.

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