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

mercurial/scmutil.py

@@ -1166,7 +1166,7 b' def dirstatecopy(ui, repo, wctx, src, ds'
             wctx.copy(origsrc, dst)
 
 def writerequires(opener, requirements):
-    with opener('requires', 'w') as fp:
+    with opener('requires', 'w', atomictemp=True) as fp:
         for r in sorted(requirements):
             fp.write("%s\n" % r)