#137 Ignore rather than delete when snapshotting
Currently vadvance removes certain "junk" files (e.g. Emacs backups, core files) before taking an immutable snapshot. However in some cases, it would be preferable to keep the files in the working directory rather than deleting them, and yet not have them appear in the immutable snapshot. We've run into cases like this before with tool-specific temporary files (lock files and a journal of edits from a CAD tool). The approach taken with these has been to automatically remove them and vadvance again before checking in, which is rather ugly (as it automates checking in something different from the last thing used in a build).
It would be nice to instead have some way to simply leave certain files/directories out of a snapshot. Such files/directories to be left out of a snapshot could be communicated to the repository server with an attribute (perhaps "#snapshot-exclude"). vadvance could also have some matching as with junk files for such "precious" files and automatically add the attribute.
