fix(git-generic) Ensure that git add works with absolute file paths

[dduportal]

Fix #366

The PRs #313 and #363 reworked the resources file and yaml with the side effect of switching the list of file path returned by their TargetFromScm() function to absolute paths, leading to error entry not found when git-adding the files in a target with scm case.

This PR fixes this behavior by treating the file path directly into the git generic plugin: if the file path that should be git added are absolute, then updatecli tries to convert these paths as relative to the scm's working directory (or raise an error otherwise).

Test

To test this pull request, you can run the following commands:

git checkout https://github.com/jenkins-infra/pipeline-library -b c5c4b22
cd pipeline-library
updatecli-next --debug apply --values ./updatecli/values.yaml --config ./updatecli/updatecli.d/docker-builder.yml

=> Tested successfully on my fork: https://github.com/dduportal/pipeline-library/pull/10

Additional Information

Tradeoff

The resources file or yaml could have been updated to fix the file path, but fixing it on the "central" git seemed safer: it removes a constraint on the plugins code for any contributor. WDYT @olblak ?

Potential improvement

The resources file or yaml could be updated to use relative path in complement of the fix.