fix race in localrepo.addchangegroup. localrepo.addchangegroup writes to changelog, then manifest, then normal files. this breaks access ordering. if reader reads changelog while manifest is being written, can find pointers into places in manifest that are not yet written. same can happen for manifest and normal files. fix is to make almost no change to localrepo.addchangegroup. it must to write changelog and manifest data early because it has to read them while writing other files. instead, write changelog and manifest data to temp file that reader cannot see, then append temp data to manifest after all normal files written, finally append temp data to changelog. temp file code is in new appendfile module. can be used in other places with small changes. much smaller race still left. we write all new data in one write call, but reader can maybe see partial update because python or os or filesystem cannot always make write really atomic. file locking no help: slow, not portable, not reliable over nfs. only real safe other plan is write to temp file every time and rename, but performance bad when manifest or changelog is big.