Add support for GIT_DIR and GIT_WORK_TREE environment variables

[splitDEV]

This Pull Request adds support for GIT_DIR and GIT_WORK_TREE environment variables.
git uses those two to set --git-dir and --work-tree (see man page of git) and gitui behaves with this PR the same.

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[extrawurst]

thanks for your contribution 🥳