Make it possible to run bootstrap on a different machine than the one it was built

- Default to trying git rev-parse for the root directory

  CARGO_MANIFEST_DIR is a path on the build machine, not the running machine.
  Don't require this to succeed, to allow building from a tarball; in that case fall back to CARGO_MANIFEST_DIR.

- Set `initial_rustc` to a path based on the path of the running executable, not CARGO_MANIFEST_DIR.

  We only reset `initial_rustc` if we're sure this isn't the working tree bootstrap was originally built in,
  since I'm paranoid that setting this in other cases will cause things to break;
  it's not clear to me when $RUSTC differs from `build/$TARGET/stage0/bin/rustc` (maybe never? but better to be sure).

  Instead, only set this when
  a) We are not using a custom rustc. If someone has specified a custom rustc we should respect their wishes.
  b) We are in a checkout of rust-lang/rust other than the one bootstrap was built in.

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