rename: add hint about --at-rev if source file doesn't exist It's quite common that users want to record copy (rename) information after committing the working copy changes (i.e. an added and a deleted file). When they try `hg mv [--after] <src> <dst>`, that just fails because the source file doesn't exist. It seems helpful if we can point them to `--at-rev=.` in this case. Differential Revision: https://phab.mercurial-scm.org/D10697