bug#2117: vc-dir slowdown on subversion due to vc-stay-local

[Dan Nicolaescu]

Using vc-dir on a big tree under subversion (like GCC for example) is
slow and does not seem to be asynchronous, emacs is unresponsive for a
long time.

The problems was introduced in revision 1.98 of vc-svn.el and is due to
this:  (let ((remote (not (vc-stay-local-p dir))))

(vc-stay-local-p DIR)  ends up calling (vc-svn-registered DIR) and that
executes and parses "svn status -v DIR" which for a big tree like gcc is
60K lines...