git svn: make minimize URL more reliable over http(s)

git svn: make minimize URL more reliable over http(s)

In addition to path-based restrictions, Subversion servers over
http(s) may have access controls implemented via the LimitExcept
directive in Apache.  In some cases, LimitExcept may be
(arguably) misconfigured to not allow REPORT requests while
allowing OPTIONS and PROPFIND.

This caused problems with our existing minimize_url logic that
only issued OPTIONS and PROPFIND requests when connecting and
using SVN::Ra::get_latest_revnum.  We now call SVN::Ra::get_log
if get_latest_revnum succeeds, resulting in a REPORT request
being sent.  This will increase our chances of tripping access
controls before we start attempting to fetch history.

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