Fix Gitlab commit range selection for merged results pipelines

[Paul-GitGuardian]

Gitlab provides "merged results pipelines" to run in merge requests, as an alternate behavior to the base MR pipelines. However, env variables take unexpected values in these pipelines. Particularly, CI_COMMIT_BEFORE_SHA has a non-empty SHA, equal to the last commit pushed. This results in empty commit ranges, meaning CI scans will not detect vulnerabilities in the MR commits.

CI_MERGE_REQUEST_TARGET_BRANCH_NAME is used here, as it is known to be working in merged results requests

[GG-HH]

Could you add a test please ?

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (2e72fa5) 92.01% compared to head (026e73f) 91.92%. Report is 9 commits behind head on main.

Files	Patch %	Lines
ggshield/core/git_hooks/ci/previous_commit.py	0.00%	2 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #803 +/- ## ========================================== - Coverage 92.01% 91.92% -0.10% ========================================== Files 157 158 +1 Lines 6586 6622 +36 ========================================== + Hits 6060 6087 +27 - Misses 526 535 +9 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/803/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/GitGuardian/ggshield/pull/803/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.92% <0.00%> (-0.10%)` | :arrow_down: | 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=GitGuardian#carryforward-flags-in-the-pull-request-comment) to find out more.

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