fix: report commit hashes instead of unrelated component tags

[kvinwang]

Problem

git_version! uses git describe, which selects the nearest tag. Because dstack is a monorepo with component-specific tags, binaries can report an unrelated component tag. For example, VMM reported git:verifier-v0.5.11-..., which the UI truncated to git:verifier-v.

Change

• add a shared dstack-build-info crate;
• expose a zero-argument git_revision!() macro with the repository-wide Git revision policy;
• exclude tags so revisions consistently use the 20-character commit hash;
• preserve the git: prefix, dirty suffix, and fallback behavior;
• migrate VMM, Gateway, KMS, Supervisor, and Guest Agent to the shared macro.

Validation

cargo fmt --all -- --check
cargo check -p dstack-build-info -p dstack-vmm -p dstack-gateway -p dstack-kms -p supervisor -p dstack-guest-agent

[Copilot]

Pull request overview

This PR fixes the VMM’s reported revision string so it always shows an abbreviated commit hash (rather than an unrelated component tag) in the version output and UI.

Changes:

• Updates the git_version! invocation to pass git describe --exclude=*, preventing tag-based descriptions in this monorepo.
• Adds a short comment explaining why tags are excluded (component-scoped tags can be misleading for VMM).

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