atomicfile: don't copy the original file if it'll be truncated

use atomictemp files to write the dirstate

use atomictemp files to write branch.cache

util.opener: if requested, use atomicfile even if the file doesn't exist Right now, surprisingly enough, if you request an atomic file but the file still doesn't exist, you get a regular file. AFAICS, the only time this happens is during the initial creation of the dirstate.