fix creating GitLab comments on unmodified lines

[alexintech]

This pull request fixes some issues with GitLab Reporter:

• get all existing comments using pagination
• fixing incorrect request new_line=0,old_line=0 if comment should be created on unmodified line that doesn't exist in the diff

Added extra tests for different cases for creating comments in GitLab.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.73684% with 2 lines in your changes missing coverage. Please review.

Project coverage is 94.73%. Comparing base (17371fe) to head (fc704ae). Report is 17 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/reporter/gitlab.go	94.73%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1147 +/- ## ========================================== - Coverage 95.13% 94.73% -0.40% ========================================== Files 103 103 Lines 10350 12110 +1760 ========================================== + Hits 9846 11473 +1627 - Misses 351 485 +134 + Partials 153 152 -1 ```

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[prymitive]

• get all existing comments using pagination

Can you split this into a dedicated commit so it's easier to review changes in this PR please?

[alexintech]

• get all existing comments using pagination

Can you split this into a dedicated commit so it's easier to review changes in this PR please?

sure, done.