Atomically write cache files

[asottile]

Resolves #673

Before:

$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'
error: cannot format x/57.py: Ran out of input
error: cannot format x/77.py: Ran out of input
$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'
error: cannot format x/75.py: Ran out of input
error: cannot format x/85.py: Ran out of input
error: cannot format x/73.py: Ran out of input
$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'
error: cannot format x/31.py: Ran out of input
error: cannot format x/35.py: Ran out of input
error: cannot format x/30.py: Ran out of input
error: cannot format x/79.py: Ran out of input
error: cannot format x/49.py: Ran out of input
error: cannot format x/23.py: Ran out of input

After:

$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'
$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'
$ find x -name "*.py" | xargs -P8 --replace bash -c 'rm -rf ~/Library/Caches/black && black -q {}'

[coveralls]

Pull Request Test Coverage Report for Build 867

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.06%) to 91.587%

---

Files with Coverage Reduction	New Missed Lines	%
black.py	2	94.66%

Totals
Change from base Build 865:	-0.06%
Covered Lines:	2787
Relevant Lines:	3043

---

💛 - Coveralls

[JelleZijlstra]

I suppose there's still a race condition, but now the effect is just that we lose some updates, instead of corrupting the cache or whatever it did before.

[JelleZijlstra]

Thanks for your PR!