subrepo/svn: fix checked out rev number retrieval (issue2968) The initial version was to take the "Revision" field from svn info. It works but produces false positive when parent paths are being moved or unrelated changes are being committed, causing it to change while the svn checkout itself remains the same. To avoid spurious commit, we took "Revision" and "Last Changed Rev" for general comparison and kept the latter to answer "what is your revision?" question. This is better but fails when the subrepo path exists at "Revision" but not at "Last Changed Rev". This patch adds a check for this, and returns "Revision" if the path does not exist. We try to avoid doing this as much as possible at it implies an extra, *remote* call.