Wrong line numbers and absolute paths in GitLab output

[qcz]

There are two issues with the GitLab output which renders the output unusable with the GitLab Code Quality UI integration:

• The first is that the line numbers are wrong (one fewer than it should be) in the GitLab output.
• The output contains absolute paths which makes file links and the code quality issue display on the changes screen not work at all. Also it should contain / as path separator insead of \

For example there is a problem in the CLI:

MyFile.cs(131,10): error CA2201: Exception type System.Exception is not sufficiently specific

In the standard XML output we can see:

<Diagnostic Id="CA2201">
    <Severity>Error</Severity>
    <Message>Do not raise reserved exception types</Message>
    <FilePath>C:\My\Repo\Path\SubDirectory\MyCsharpFile.cs</FilePath>
    <Location Line="131" Character="10" />
</Diagnostic>

In the Gitlab output however:

{
    "type": "issue",
    "fingerprint": "2af2bff19cc84a89e8f603cd2acaf3150dbe2a8504751eb6ac8635dac24a7590",
    "check_name": "CA2201",
    "description": "Do not raise reserved exception types",
    "severity": "critical",
    "location": {
      "path": "C:\\My\\Repo\\Path\\SubDirectory\\MyCsharpFile.cs",
      "lines": {
        "begin": 130
      }
    },
    "categories": [
      "Usage"
    ]
  },

Note the wrong line number and the absolute path. The correct content for GitLab should be (if the repo is at C:\My\Repo\Path):

{
    "type": "issue",
    "fingerprint": "2af2bff19cc84a89e8f603cd2acaf3150dbe2a8504751eb6ac8635dac24a7590",
    "check_name": "CA2201",
    "description": "Do not raise reserved exception types",
    "severity": "critical",
    "location": {
      "path": "SubDirectory/MyCsharpFile.cs",
      "lines": {
        "begin": 131
      }
    },
    "categories": [
      "Usage"
    ]
  },

[josefpihrt]

cc: @marais-vzc