Building from rustc-beta-src.tar.xz fails on git commands

[cuviper]

I'm trying to test Fedora builds of the current beta:

$ cat rustc-beta-src/version
1.24.0-beta.8 (ed9751a90 2018-01-23)

Rustbuild fails in Build::beta_prerelease_version():

failed to execute command: "git" "ls-remote" "origin" "beta"
error: No such file or directory (os error 2)

With network access, git ls-remote could work with a URL instead of the missing origin remote. However, the configuration we use for rpm builds is completely offline. (Actually, I probably don't even have git in the build root at the moment, but I could add that.)

A few options:

• Just don't support --release-channel=beta with non-git sources. (status quo)
• If git fails, call the release X.Y.Z-beta without a prerelease version at all.
• Try to extract the prerelease version from the same version file I quoted above.
• Allow an environment variable or config option to set a prerelease version manually.

[kennytm]

cc @alexcrichton

[alexcrichton]

Oops, sorry about that! I think even if we had a url it'd fail later because it wants to find the diff between the origin's branches and the local revision, but with the tarballs there's no local revision!

I'd say we should just drop the prerelease number entirely from betas-build-from-tarballs perhaps? Having an optional env var override seems fine by me though!