hook install adds a merge attribute for gitignored graphify-out/graph.json

[MohabMohie]

Reproduction

On Graphify 0.9.38:

1. A repository ignores /graphify-out/.
2. Its tracked .gitattributes has no Graphify rule.
3. Run graphify hook install.

The installer appends:

graphify-out/graph.json merge=graphify

This dirties the tracked worktree even though git check-ignore graphify-out/graph.json confirms the file can never participate in a merge.

Re-running graphify hook install after removing the redundant line adds it again. The executable hooks themselves append safely and work.

Expected

If graphify-out/graph.json is ignored, register the local merge-driver config but do not mutate tracked .gitattributes, or make the attribute opt-in. Preserve current behavior when graph output is tracked.

This matters for automated tool upgrades because the official guidance says to rerun graphify hook install after reinstall/upgrade; an otherwise local hook refresh should not create an unrelated tracked diff.

[jecanore]

Ran into this from the other direction while setting up hook install on a project with a customized output directory — filed #3206 for the wrong-path part. Both bugs live in the same _register_merge_driver/_merge_attr_line code, so I opened #3207 to fix them together (skips the .gitattributes line when the target is gitignored, exactly what's described here).