git-svn: correctly handle do_{switch,update} in deep directories

git-svn: correctly handle do_{switch,update} in deep directories

The do_update or do_switch functions in SVN only allow for a
single path component; so 'path/to/deep/dir' would be
interpreted as 'path'.

SVN 1.4.x has a reparent function that can let us change the
session to use a higher-level root of the repository, so we can
use that for do_switch (which still doesn't seem to work in SVN
1.4.3 (a fix was attempted, but they missed the rest of the
typemap changes needed in trunk...)).

On the do_update side, we can use set_path on higher level
directories and set them to a newer revision so they don't get
updated.  We can't do this with do_switch, either, because the
relative path we're tracking can change (directory moving into
a child of itself).

Because of these changes, we need to double check that our Fetch
editor is correctly performing stripping on any prefixed paths
from update, otherwise we'll just die() because that would be
a bug.

Added a test case which helped me notice and fix problems with
do_switch, too.

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