Obtain git-version from macro, instead of custom build-script

[Urhengulas]

This implements the idea from @japaric to utilize the git_version crate for obtaining the git hash, instead of the custom build-script we used so far.

In case git is not present on the system the reported version will only consist of the CARGO_PKG_VERSION.

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_note: I am a bit unsure where exactly to put the extract_git_hash helper function. I mainly considered the utils module. I decided to put it into main.rs for now to not potentially conflict with the upcoming refactoring from @Lotterleben and @japaric._

[Lotterleben]

main.rs will be heavily refactored, but this PR shouldn't be too painful to rebase onto the refactored code I think. If that*#s okay with you, I'd like to wait with merging this PR until after the refactoring is done.

[Urhengulas]

main.rs will be heavily refactored, but this PR shouldn't be too painful to rebase onto the refactored code I think. If that*#s okay with you, I'd like to wait with merging this PR until after the refactoring is done.

Sure!

[Urhengulas]

@Lotterleben I've rebased it onto the latest main.

[Lotterleben]

feel free to bors rplus

[Urhengulas]

bors r+

[bors[bot]]

Build succeeded:

• test (nightly, macOS-latest)
• test (nightly, ubuntu-latest)
• test (nightly, windows-latest)
• test (stable, macOS-latest)
• test (stable, ubuntu-latest)
• test (stable, windows-latest)