JGit does not commit files in fileset based in subfolder

If `mvn release` is run in a subfolder, the changed files are relative to the subfolder and not the repo root and `maven-scm-provider-jgit` does not compute the correct path and thus the files are not added and committed.

For example, a git repo is created at `repo-root`

repo-root/pom.xml
repo-root/unparented-submodule/pom.xml 

The user does:

cd repo-root/unparented-submodule
mvn release:prepare 

A `git status` will show that `unparented-submodule/pom.xml` is changed and not staged for commit.  That's because the fileset's basedir is `unpainted-submodule` and the relative path is just `pom.xml`. `maven-scm-provider-jgit` 1.12.1 tries to stage 'pom.xml' not `unparented-submodule/pom.xml` and since repo-root/pom.xml has not changed, nothing is in the commit and `unparented-submodule/pom.xml` remains un-staged.