fix(iac): prepush scan all

[carla-gitguardian]

This MR aims to fix 2 edge cases during IaC pre-push scans:

• Use scan diff (instead of all) when pushing a new branch. The ref commit should be the last one on the parent branch.
• Prevent staged files to be scanned during pre-push "all" scans. It should only occurs on the first push for each repo. This avoids to push a vulnerability that could have been fixed in the staged files only.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (5a6f8a5) 91.58% compared to head (f4b1ff2) 91.61%. Report is 3 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #824 +/- ## ========================================== + Coverage 91.58% 91.61% +0.02% ========================================== Files 167 168 +1 Lines 6896 6920 +24 ========================================== + Hits 6316 6340 +24 Misses 580 580 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/824/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/824/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `91.61% <100.00%> (+0.02%)` | :arrow_up: | 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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