cmd/save: when a file is chunked, mangle its name from * to *.bup

cmd/save: when a file is chunked, mangle its name from * to *.bup

Files that are already named *.bup are renamed to *.bup.bupl, so that we can
just always drop either .bup or .bupl from a filename if it's there, and the
result will be the original filename.

Also updated lib/bup/vfs.py to demangle the names appropriately, and treat
git trees named *.bup as real chunked files (ie. by joining them back
together).