Link cross-project cross-reference notes to correct project.
- Fixes #1431: Don't crash when an MR from a fork has a cross-reference comment from the target project on of its commits.
- Include commit comments in MR from a forked project.
To accomplish this, the Commit model needs to know about its project. Adding this attribute also simplifies a bunch of other methods that deal with a note's target, that previously had to have special logic for when it belonged to a commit.
See merge request !554
