fix(transformer): find last modified date form commit on submodule (#1831)

* fix(transformer): find last modified date form commit on submodule

when the content folder has a submodule git, the relative path start in content folder and not root folder of quartz

* fix(transformer): use path.relative for improved path handling in last modified date calculation

* fix(transformer): keep find file from relative path of repo workdir

* fix(transformer): use variable for repository workdir

use default value if repo.workdir is undefined to user fullFp value