rewriteutil: move publicrevs closer to where it's used

requires: use atomictemp=True when writing .hg/requires We use an unusual file system at Google that allows writes (and renames) but not deletions (for certain paths). That causes problems when writing the requires files without atomictemp=True. There doesn't appear to be any real drawbacks to using atomictemp, so I'm hoping we can just change it in core. Differential Revision: https://phab.mercurial-scm.org/D5274