When a project is opened through a symlinked path, libgit2's
git_repository_workdir() returns the canonical path with symlinks
resolved. This caused a path mismatch between the tree cache
(which uses the user-provided symlinked path) and git status
results (which used canonical paths), preventing status icons
from being displayed.
The fix introduces SetProjectDir()/GetProjectDir() methods to
KIGIT_COMMON to store and use the user-provided project path.
A helper function computeSymlinkPreservingWorkDir() computes
the git working directory path while preserving symlinks from
the original user path.
Fixes https://gitlab.com/kicad/code/kicad/-/issues/21092
Wayne Stambaugh