Can't find commit range for nightly toolchain

[Stupremee]

cargo-bisect-rustc is unable to find the commit range for a nightly toolchain, if the git commit hash file of the previous nightly returns 404.

If cargo-bisect-rustc is trying to find the commit range for nightly-2020-09-02, it will step one day back (2020-09-01) but this nightly doesn't have a statically hosted git commit hash and thus cargo-bisect-rustc fails.

It should try to step back more versions until it finds one that doesn't return 404 (2020-08-29 in this case)

[tesuji]

Related #112