attr: teach "--attr-source=<tree>" global option to "git"

Earlier, 47cfc9bd (attr: add flag --source to work with tree-ish, 2023-01-14) taught "git check-attr" the "--source=" option to allow it to read attribute files from a tree-ish, but did so only for the command. Just like "check-attr" users wanted a way to use attributes from a tree-ish and not from the working tree files, users of other commands (like "git diff") would benefit from the same.

Undo most of the UI change the commit made, while keeping the internal logic to read attributes from a given tree-ish. Expose the internal logic via a new "--attr-source=" command line option given to "git", so that it can be used with any git command that runs internally.

Add an environment variable GIT_ATTR_SOURCE that is set when --attr-source is passed in, so that subprocesses use the same value for the attributes source tree.