[BTG-32] Fix git revision inclusion

Description

Problem: in 'DGitRevision' documentation item I called template haskell which fetches commit sha and date internally rather than at call site, and this is obviously incorrect as can observed in depending projects.

Solution: make 'mkDGitRevision' function return TH expression which yet should be instantiated.

Related issue(s)

https://issues.serokell.io/issue/BTG-32

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Related changes (conditional)

• Tests (see short guidelines)

  • If I added new functionality, I added tests covering it.
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    • README
    • Haddock
    • docs/

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