largefiles: respect store.createmode and avoid extra file copy
Before, a tempfile was used to create a temp file was created with 600
permissions and the uploaded data was written into it. This file was
then *copied* to .hg/largefiles/<hash>.
We now simply use atomictempfile to write the data to a temp file with
the right permissions and then rename that into place.
$ hg init t
$ cd t
$ echo a > a
$ hg add a
$ hg commit -m test
$ rm .hg/requires
$ hg tip
abort: index 00changelog.i unknown format 2!
[255]
$ echo indoor-pool > .hg/requires
$ hg tip
abort: unknown repository format: requires features 'indoor-pool' (upgrade Mercurial)!
[255]
$ echo outdoor-pool >> .hg/requires
$ hg tip
abort: unknown repository format: requires features 'indoor-pool', 'outdoor-pool' (upgrade Mercurial)!
[255]