Check exit status of git commands spawned by build script

[dtolnay]

The git commands git rev-parse HEAD and git log -1 --date=short --pretty=format:%cd in rustfmt's build script might fail with "fatal: not a git repository (or any of the parent directories): .git" if rustfmt 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.

Previously, rustfmt's build script would take that empty stdout and treat it like valid commit info, unlike what it does when git cannot be spawned in the first place, and contradicting this comment about the build script's intended behavior in the case of there not being a git repository:

rustfmt/build.rs

Lines 24 to 25 in 17c5869

	// Try to get hash and date of the last commit on a best effort basis. If anything goes wrong
	// (git not installed or if this is not a git repository) just return an empty string.

$ curl -L https://github.com/rust-lang/rustfmt/archive/refs/heads/master.tar.gz | tar xz
$ cd rustfmt-master/
$ cargo run --bin rustfmt -- --version

Before this PR: rustfmt 1.7.1-nightly ( )
After first commit: rustfmt 1.7.1-
After second commit: rustfmt 1.7.1

[ytmimi]

Thanks! This definitely feels like the right output in case we can't grab additional info from git.