subrepo/svn: cache _wcrev() value in merge()

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.

amend: fix copy records handling (issue3410) Messing with the dirstate before the intermediate commit seems error prone. Instead, commit and recompute the copies with copies.pathcopies(), then use that with commitctx(). Since copies.pathcopies() does not support file replacement very well, the whole .renamed() condition in samefile() is removed and the "file replacement caused by differing copy source" effect is discarded. Test shamelessly stolen from Idan Kamara <idankk86@gmail.com>