Fix IAC scan all on subdirectories

[Paul-GitGuardian]

Currently, tars have two ways to be created in GGShield:

• For iac/sca scan diff, filepaths are listed using git ls-tree --full-name, which return filepaths relative to the git root
• For iac/sca scan all, filepaths are listed using directory.rglob("*") (where directory is the argument passed to the command), then returned relative to the directory using path.relative_to(directory)
• The difference comes from the necessity of listing files for a given git reference, for diff scans

However, the all behavior introduces two issues for IAC when calling the command with a Git subdirectory as argument:

1. File contents are not shown in the output, as the output handler looks for files relative to the git root, but is given filepaths relative to the subdir argument
2. Only the subdir is added to the tar sent to the API, preventing from linking existing incidents properly (as their paths are relative to the git root) SCA is not impacted by these issues yet, but would be impacted by 2. when implemented.

This PR solves this issue by using the git root for all scans too as the root directory. If the scanned directory isn't a Git repository, the current behavior is maintained.

[codecov-commenter]

Codecov Report

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

Comparison is base (8eb47db) 91.96% compared to head (056864d) 92.04%. Report is 4 commits behind head on main.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #820 +/- ## ========================================== + Coverage 91.96% 92.04% +0.07% ========================================== Files 160 160 Lines 6786 6788 +2 ========================================== + Hits 6241 6248 +7 + Misses 545 540 -5 ``` | [Flag](https://app.codecov.io/gh/GitGuardian/ggshield/pull/820/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/820/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=GitGuardian) | `92.04% <100.00%> (+0.07%)` | :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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[Paul-GitGuardian]

For SCA, after the fix, the output will show filepaths relative to the git repository root, instead of relative to the given directory. Is that a problem?