@Andrzej-Jaworski said:
How commands SVN Diff and GIT Diff work?
Again, the topic is more appropriate for Plugin Development category.
As @Eko-palypse already answered those commands are used to compare file against it’s commited Git/SVN revision controlled version.
As @Eko-palypse already said Compare plugin can only show the file’s difference (unstaged/uncommited) against the Git/SVN repo.
It is not a revision control plugin.