Auto merge of #13217 - dtolnay-contrib:toolsutil, r=flip1995

Check exit status of subcommands spawned by rustc_tools_util

The git commands `git rev-parse --short HEAD` and `git log -1 --date=short --pretty=format:%cd` that clippy runs from its build script might fail with **"fatal: not a git repository (or any of the parent directories): .git"** if clippy is being built from a source tarball rather than a git repository. That message is written by git to stderr, and nothing is written to stdout.

For `clippy-driver --version` this PR wouldn't make a difference because it treats empty stdout and failed spawns (`git` is not installed) identically:

https://github.com/rust-lang/rust-clippy/blob/7ac242c3d0d3ee867a6c9cdcbdec986c408ac36f/rustc_tools_util/src/lib.rs#L35-L42

But other users of `rustc_tools_util` should be able to expect that the distinction between Some and None is meaningful. They shouldn't need extra code to handle None vs Some-and-empty vs Some-and-nonempty.

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changelog: none

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