Use overlay FS when building projects #144
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Currently, we install a repo, then run TS on it, then run TS on it again. This creates a situation where the results of the first compile can negatively affect the second.
In #131, I attempted to fix this by using git to stage all files after installing (even if ignored), then reverting them. That didn't work because FS/memory cost of that was too high for the runner.
Instead, we can use OverlayFS to do this; we install into the lower layer, then perform all operations on an upper layer. After the first run, we throw away the overlay and create a new one, such that the second compilation sees the repo as being freshly installed.
If tmpfs is disabled, we instead use multiple directories and just reflink copy them. Theoretically we could use copying within the tmpfs too, but I don't believe that tmpfs can do that.
This PR is best viewed without whitespace due to indent changes: https://github.com/microsoft/typescript-error-deltas/pull/144/files?w=1
(Redo of #143)