[SDTEST-878] Optimise LocalRepository.relative_to_root helper to make test impact analysis faster

[anmarchenko]

What does this PR do? Applies some heuristics to avoid calling Pathname#relative_path_from function when determining what would be path relative to the git root for the given filepath:

• If given filepath is absolute, we use the fact that absolute paths are pre-filtered already and they always contain root as prefix, so we only need to slice the string to get the suffix (path[prefix_length..])
• If given filepath is relative, we compute Pathname#relative_path_from once and then we store the difference between the relative filepath and the computed relative path from root in the @prefix_to_root cached variable. We can do it because all relative paths are computed relative to the Dir.pwd which doesn't change in the lifetime of the process. After we have @prefix_to_root cache, we prepend it to every filepath we get (or just return the filepath if the tests are running from the git root).

Motivation Profiling rubocop test suite with Datadog continuous profiler showed that big part of CPU time is spent in Datadog::CI::Git::LocalRepository.relative_to_root function:

How to test the change? Unit tests are provided.

Benchmark results:

• Middleman overhead 34,1% (down from 40%)
• Rubocop overhead 64% (down from 128%) - 2x improvement

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.62500% with 3 lines in your changes missing coverage. Please review.

Project coverage is 98.86%. Comparing base (7d381d5) to head (777af47).

Files with missing lines	Patch %	Lines
lib/datadog/ci/git/local_repository.rb	93.33%	1 Missing :warning:
lib/datadog/ci/transport/http.rb	50.00%	1 Missing :warning:
...atadog/ci/test_optimisation/coverage/event_spec.rb	93.33%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #244 +/- ## ========================================== - Coverage 98.88% 98.86% -0.02% ========================================== Files 279 279 Lines 13575 13603 +28 Branches 625 637 +12 ========================================== + Hits 13424 13449 +25 - Misses 151 154 +3 ``` | [Flag](https://app.codecov.io/gh/DataDog/datadog-ci-rb/pull/244/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/DataDog/datadog-ci-rb/pull/244/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog) | `?` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog#carryforward-flags-in-the-pull-request-comment) to find out more.

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