fix: Staging fails when cwd differs from repo root

[MaxFangX]

When GitFile(filename) was created with a relative path, git_repo.discover() resolved it relative to the current working directory. If the user :cd'd to a subdirectory, paths got resolved incorrectly and staging operations silently failed.

• DiffScreen: Use self.git_file instead of creating new GitFile
• ProjectDiffScreen: Construct absolute paths using stored reponame
• Remove redundant filename params from Model methods
• Remove redundant git_repo.discover() calls (use stored reponame)
• Fix extra unused arg in git_file:unstage(filename)