give a better error when origin/HEAD is vastly out of date

[jyn514]

i just tried to compile a recent branch of my fork and got this confusing error:

downloading https://ci-artifacts.rust-lang.org/rustc-builds/3572d7451d1062e8bccf310af6bbf6255091d296/rust-dev-nightly-x86_64-pc-windows-msvc.tar.xz
curl: (22) The requested URL returned error: 404

Fallback to PowerShell
Exception calling "DownloadFile" with "2" argument(s): "The remote server returned an 
error: (404) Not Found."
At line:1 char:82
+ ... pe]::Tls12; (New-Object System.Net.WebClient).DownloadFile('https://c ...
+                 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [], MethodInvocationException
    + FullyQualifiedErrorId : WebException
 
thread 'main' panicked at src\core\download.rs:211:9:
std::fs::rename(&tempfile, dest_path) failed with The system cannot find the file specified. (os error 2)

notice that 3572d74 is from june. this is because git merge-base origin/master HEAD outputs 69a6373, which in turn is because origin/master points to 69a6373, which is 6 months old.

it would be nice for bootstrap to give a warning if origin/master is massively outdated. anything more than a week is probably suspect, or if there's any commits in git log origin/master..HEAD that aren't authored by the current user.

there is a second error here that apparently the help_on_error reporting only works on linux, not windows.

@rustbot label +A-diagnostics +T-bootstrap

[Teapot4195]

On my first attempt it outputted spurious failure, trying again a good number of times and then did in fact output a help message. But when trying again, it then panicked with thread 'main' panicked at 'member failed with premature eof', download.rs:278:30 instead.

I think I have an idea as to how to fix this issue, perhaps removing the build/cache/llvm-xxxx-false folder when the download fails so the error is consistent, I'm a little bit unsure about this approach, so if I could get some comments it would be greatly appreciated.

@rustbot claim